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Filename : DM5.pod
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=pod

=head1 NAME

Date::Manip::DM5 - Date manipulation routines

=head1 SYNOPSIS

 use Date::Manip;

 $version = DateManipVersion;

 Date_Init();
 Date_Init("VAR=VAL","VAR=VAL",...);
 @list = Date_Init();
 @list = Date_Init("VAR=VAL","VAR=VAL",...);

 $date = ParseDate(\@args);
 $date = ParseDate($string);
 $date = ParseDate(\$string);

 @date = UnixDate($date,@format);
 $date = UnixDate($date,@format);

 $delta = ParseDateDelta(\@args);
 $delta = ParseDateDelta($string);
 $delta = ParseDateDelta(\$string);

 @str = Delta_Format($delta,$dec,@format);
 $str = Delta_Format($delta,$dec,@format);

 $recur = ParseRecur($string,$base,$date0,$date1,$flags);
 @dates = ParseRecur($string,$base,$date0,$date1,$flags);

 $flag = Date_Cmp($date1,$date2);

 $d = DateCalc($d1,$d2 [,$errref] [,$del]);

 $date = Date_SetTime($date,$hr,$min,$sec);
 $date = Date_SetTime($date,$time);

 $date = Date_SetDateField($date,$field,$val [,$nocheck]);

 $date = Date_GetPrev($date,$dow,$today,$hr,$min,$sec);
 $date = Date_GetPrev($date,$dow,$today,$time);

 $date = Date_GetNext($date,$dow,$today,$hr,$min,$sec);
 $date = Date_GetNext($date,$dow,$today,$time);

 $name = Date_IsHoliday($date);

 $listref = Events_List($date);
 $listref = Events_List($date0,$date1);

 $date = Date_ConvTZ($date);
 $date = Date_ConvTZ($date,$from);
 $date = Date_ConvTZ($date,"",$to);
 $date = Date_ConvTZ($date,$from,$to);

 $flag = Date_IsWorkDay($date [,$flag]);

 $date = Date_NextWorkDay($date,$off [,$flag]);

 $date = Date_PrevWorkDay($date,$off [,$flag]);

 $date = Date_NearestWorkDay($date [,$tomorrowfirst]);

The above routines all check to make sure that Date_Init is called.  If it
hasn't been, they will call it automatically.  As a result, there is usually
no need to call Date_Init explicitly unless you want to change some of the
config variables (described below).  They also do error checking on the
input.

The routines listed below are intended primarily for internal use by other
Date::Manip routines.  They do little or no error checking, and do not
explicitly call Date_Init.  Those functions are all done in the main
Date::Manip routines above.

Because they are significantly faster than the full Date::Manip routines,
they are available for use with a few caveats.  Since little or no
error checking is done, it is the responsibility of the programmer to
ensure that valid data (AND valid dates) are passed to them.  Passing
invalid data (such as a non-numeric month) or invalid dates (Feb 31)
will fail in unpredictable ways (possibly returning erroneous results).
Also, since Date_Init is not called by these, it must be called
explicitly by the programmer before using these routines.

In the following routines, $y may be entered as either a 2 or 4 digit year
(it will be converted to a 4 digit year based on the variable YYtoYYYY
described below).  Month and day should be numeric in all cases.  Most (if
not all) of the information below can be gotten from UnixDate which is
really the way I intended it to be gotten, but there are reasons to use
these (these are significantly faster).

 $day = Date_DayOfWeek($m,$d,$y);
 $secs = Date_SecsSince1970($m,$d,$y,$h,$mn,$s);
 $secs = Date_SecsSince1970GMT($m,$d,$y,$h,$mn,$s);
 $days = Date_DaysSince1BC($m,$d,$y);
 $day = Date_DayOfYear($m,$d,$y);
 ($y,$m,$d,$h,$mn,$s) = Date_NthDayOfYear($y,$n);
 $days = Date_DaysInYear($y);
 $days = Date_DaysInMonth($m,$y);
 $wkno = Date_WeekOfYear($m,$d,$y,$first);
 $flag = Date_LeapYear($y);
 $day = Date_DaySuffix($d);
 $tz = Date_TimeZone();

=head1 ROUTINES

=over 4

=item B<Date_Init>

 Date_Init();
 Date_Init("VAR=VAL","VAR=VAL",...);
 @list = Date_Init();
 @list = Date_Init("VAR=VAL","VAR=VAL",...);

Normally, it is not necessary to explicitly call Date_Init.  The first
time any of the other routines are called, Date_Init will be called to set
everything up.  If for some reason you want to change the configuration of
Date::Manip, you can pass the appropriate string or strings into Date_Init
to reinitialize things.

The strings to pass in are of the form "VAR=VAL".  Any number may be
included and they can come in any order.  VAR may be any configuration
variable.  A list of all configuration variables is given in the section
CUSTOMIZING DATE::MANIP below.  VAL is any allowed value for that variable.
For example, to switch from English to French and use non-US format (so
that 12/10 is Oct 12), do the following:

  Date_Init("Language=French","DateFormat=non-US");

If Date_Init is called in list context, it will return a list of all
config variables and their values suitable for passing in to Date_Init
to return Date::Manip to the current state.  The only possible problem is
that by default, holidays will not be erased, so you may need to prepend
the "EraseHolidays=1" element to the list.

=item B<ParseDate>

 $date = ParseDate(\@args);
 $date = ParseDate($string);
 $date = ParseDate(\$string);

This takes an array or a string containing a date and parses it.  When the
date is included as an array (for example, the arguments to a program) the
array should contain a valid date in the first one or more elements
(elements after a valid date are ignored).  Elements containing a valid
date are shifted from the array.  The largest possible number of elements
which can be correctly interpreted as a valid date are always used.  If a
string is entered rather than an array, that string is tested for a valid
date.  The string is unmodified, even if passed in by reference.

The real work is done in the ParseDateString routine.

The ParseDate routine is primarily used to handle command line arguments.
If you have a command where you want to enter a date as a command line
argument, you can use Date::Manip to make something like the following
work:

  mycommand -date Dec 10 1997 -arg -arg2

No more reading man pages to find out what date format is required in a
man page.

Historical note: this is originally why the Date::Manip routines were
written (though long before they were released as the Date::Manip module).
I was using a bunch of programs (primarily batch queue managers) where
dates and times were entered as command line options and I was getting
highly annoyed at the many different (but not compatible) ways that they
had to be entered.  Date::Manip originally consisted of basically 1 routine
which I could pass "@ARGV" to and have it remove a date from the beginning.

=item B<ParseDateString>

 $date = ParseDateString($string);

This routine is called by ParseDate, but it may also be called directly
to save some time (a negligible amount).

NOTE:  One of the most frequently asked questions that I have gotten
is how to parse seconds since the epoch.  ParseDateString cannot simply
parse a number as the seconds since the epoch (it conflicts with some
ISO-8601 date formats).  There are two ways to get this information.
First, you can do the following:

    $secs = ...         # seconds since Jan 1, 1970  00:00:00 GMT
    $date = DateCalc("Jan 1, 1970  00:00:00 GMT","+ $secs");

Second, you can call it directly as:

    $date = ParseDateString("epoch $secs");

To go backwards, just use the "%s" format of UnixDate:

    $secs = UnixDate($date,"%s");

A full date actually includes 2 parts: date and time.  A time must include
hours and minutes and can optionally include seconds, fractional seconds,
an am/pm type string, and a time zone.  For example:

     [at] HH:MN              [Zone]
     [at] HH:MN         [am] [Zone]
     [at] HH:MN:SS      [am] [Zone]
     [at] HH:MN:SS.SSSS [am] [Zone]
     [at] HH            am   [Zone]

Hours can be written using 1 or 2 digits, but the single digit form may
only be used when no ambiguity is introduced (i.e. when it is not
immediately preceded by a digit).

A time is usually entered in 24 hour mode, but 12 hour mode can be used
as well if AM/PM are entered (AM can be entered as AM or A.M. or other
variations depending on the language).

Fractional seconds are also supported in parsing but the fractional part is
discarded (with NO rounding occurring).

Time zones always appear immediately after the time.  A number of different
forms are supported (see the section TIME ZONES below).

Incidentally, the time is removed from the date before the date is parsed,
so the time may appear before or after the date, or between any two parts
of the date.

Valid date formats include the ISO 8601 formats:

   YYYYMMDDHHMNSSF...
   YYYYMMDDHHMNSS
   YYYYMMDDHHMN
   YYYYMMDDHH
   YY-MMDDHHMNSSF...
   YY-MMDDHHMNSS
   YY-MMDDHHMN
   YY-MMDDHH
   YYYYMMDD
   YYYYMM
   YYYY
   YY-MMDD
   YY-MM
   YY
   YYYYwWWD      ex.  1965-W02-2
   YYwWWD
   YYYYDOY       ex.  1965-045
   YYDOY

In the above list, YYYY and YY signify 4 or 2 digit years, MM, DD, HH, MN, SS
refer to two digit month, day, hour, minute, and second respectively.  F...
refers to fractional seconds (any number of digits) which will be ignored.
In all cases, the date and time parts may be separated by the letter "T"
(but this is optional), so
   2002-12-10-12:00:00
   2002-12-10T12:00:00
are identical.

The last 4 formats can be explained by example:  1965-w02-2 refers to Tuesday
(day 2) of the 2nd week of 1965.  1965-045 refers to the 45th day of 1965.

In all cases, parts of the date may be separated by dashes "-".  If this is
done, 1 or 2 digit forms of MM, DD, etc. may be used.  All dashes are
optional except for those given in the table above (which MUST be included
for that format to be correctly parsed).  So 19980820, 1998-0820,
1998-08-20, 1998-8-20, and 199808-20 are all equivalent, but that date may
NOT be written as 980820 (it must be written as 98-0820).

NOTE:  Even though not allowed in the standard, the time zone for an ISO-8601
date is flexible and may be any of the time zones understood by Date::Manip.

Additional date formats are available which may or may not be common including:

  MM/DD  **
  MM/DD/YY  **
  MM/DD/YYYY  **

  mmmDD       DDmmm                   mmmYYYY/DD     mmmYYYY
  mmmDD/YY    DDmmmYY     DD/YYmmm    YYYYmmmDD      YYYYmmm
  mmmDDYYYY   DDmmmYYYY   DDYYYYmmm   YYYY/DDmmm

Where mmm refers to the name of a month.  All parts of the date can be
separated by valid separators (space, "/", or ".").  The separator "-" may
be used as long as it doesn't conflict with an ISO 8601 format, but this
is discouraged since it is easy to overlook conflicts.  For example, the
format MM/DD/YY is just fine, but MM-DD-YY does not work since it conflicts
with YY-MM-DD.  To be safe, if "-" is used as a separator in a non-ISO
format, they should be turned into "/" before calling the Date::Manip
routines.  As with ISO 8601 formats, all separators are optional except for
those given as a "/" in the list above.

** Note that with these formats, Americans tend to write month first, but
many other countries tend to write day first.  The latter behavior can be
obtained by setting the config variable DateFormat to something other than
"US" (see CUSTOMIZING DATE::MANIP below).

Date separators are treated very flexibly (they are converted to spaces),
so the following dates are all equivalent:

   12/10/1965
   12-10 / 1965
   12 // 10 -. 1965

In some cases, this may actually be TOO flexible, but no attempt is made to
trap this.

Years can be entered as 2 or 4 digits, days and months as 1 or 2 digits.
Both days and months must include 2 digits whenever they are immediately
adjacent to another numeric part of the date or time.  Date separators
are required if single digit forms of DD or MM are used.  If separators
are not used, the date will either be unparsable or will get parsed
incorrectly.

Miscellaneous other allowed formats are:
  which dofw in mmm in YY      "first Sunday in June
                               1996 at 14:00" **
  dofw week num YY             "Sunday week 22 1995" **
  which dofw YY                "22nd Sunday at noon" **
  dofw which week YY           "Sunday 22nd week in
                               1996" **
  next/last dofw               "next Friday at noon"
  next/last week/month         "next month"
  in num days/weeks/months     "in 3 weeks at 12:00"
  num days/weeks/months later  "3 weeks later"
  num days/weeks/months ago    "3 weeks ago"
  dofw in num week             "Friday in 2 weeks"
  in num weeks dofw            "in 2 weeks on Friday"
  dofw num week ago            "Friday 2 weeks ago"
  num week ago dofw            "2 weeks ago Friday"
  last day in mmm in YY        "last day of October"
  dofw                         "Friday" (Friday of
                               current week)
  Nth                          "12th", "1st" (day of
                               current month)
  epoch SECS                   seconds since the epoch
                               (negative values are
                               supported)

** Note that the formats "Sunday week 22" and "22nd Sunday" give very
different behaviors.  "Sunday week 22" returns the Sunday of the 22nd week
of the year based on how week 1 is defined.  ISO 8601 defines week one to
contain Jan 4, so "Sunday week 1" might be the first or second Sunday of
the current year, or the last Sunday of the previous year.  "22nd Sunday"
gives the actual 22nd time Sunday occurs in a given year, regardless of the
definition of a week.

Note that certain words such as "in", "at", "of", etc. which commonly appear
in a date or time are ignored.  Also, the year is always optional.

In addition, the following strings are recognized:
  today     (exactly now OR today at a given time if a time is specified)
  now       (synonym for today)
  yesterday (exactly 24 hours ago unless a time is specified)
  tomorrow  (exactly 24 hours from now unless a time is specified)
  noon      (12:00:00)
  midnight  (00:00:00)
Other languages have similar (and in some cases additional) strings.

Some things to note:

All strings are case insensitive.  "December" and "DEceMBer" both work.

When a part of the date is not given, defaults are used: year defaults
to current year; hours, minutes, seconds to 00.

The year may be entered as 2 or 4 digits.  If entered as 2 digits, it will
be converted to a 4 digit year.  There are several ways to do this based on
the value of the YYtoYYYY variable (described below).  The default behavior
it to force the 2 digit year to be in the 100 year period CurrYear-89 to
CurrYear+10.  So in 1996, the range is [1907 to 2006], and the 2 digit year
05 would refer to 2005 but 07 would refer to 1907.  See CUSTOMIZING
DATE::MANIP below for information on YYtoYYYY for other methods.

Dates are always checked to make sure they are valid.

In all of the formats, the day of week ("Friday") can be entered anywhere
in the date and it will be checked for accuracy.  In other words,
  "Tue Jul 16 1996 13:17:00"
will work but
  "Jul 16 1996 Wednesday 13:17:00"
will not (because Jul 16, 1996 is Tuesday, not Wednesday).  Note that
depending on where the weekday comes, it may give unexpected results when
used in array context (with ParseDate).  For example, the date
("Jun","25","Sun","1990") would return June 25 of the current year since
Jun 25, 1990 is not Sunday.

The times "12:00 am", "12:00 pm", and "midnight" are not well defined.  For
good or bad, I use the following convention in Date::Manip:
  midnight = 12:00am = 00:00:00
  noon     = 12:00pm = 12:00:00
and the day goes from 00:00:00 to 23:59:59.  In other words, midnight is the
beginning of a day rather than the end of one.  The time 24:00:00 is also
allowed (though it is automatically transformed to 00:00:00 of the following
day).

The format of the date returned is YYYYMMDDHH:MM:SS.  The advantage of this
time format is that two times can be compared using simple string comparisons
to find out which is later.  Also, it is readily understood by a human.
Alternate forms can be used if that is more convenient.  See Date_Init below
and the config variable Internal.

NOTE: The format for the date is going to change at some point in the future
to YYYYMMDDHH:MN:SS+HHMN*FLAGS.  In order to maintain compatibility, you
should use UnixDate to extract information from a date, and Date_Cmp to compare
two dates.  The simple string comparison will only work for dates in the same
time zone.

=item B<UnixDate>

 @date = UnixDate($date,@format);
 $date = UnixDate($date,@format);

This takes a date and a list of strings containing formats roughly
identical to the format strings used by the UNIX date(1) command.  Each
format is parsed and an array of strings corresponding to each format is
returned.

$date may be any string that can be parsed by ParseDateString.

The format options are:

 Year
     %y     year                     - 00 to 99
     %Y     year                     - 0001 to 9999
 Month, Week
     %m     month of year            - 01 to 12
     %f     month of year            - " 1" to "12"
     %b,%h  month abbreviation       - Jan to Dec
     %B     month name               - January to December
 Day
     %j     day of the year          - 001 to 366
     %d     day of month             - 01 to 31

     %e     day of month             - " 1" to "31"
     %v     weekday abbreviation     - " S"," M"," T"," W","Th"," F","Sa"
     %a     weekday abbreviation     - Sun to Sat
     %A     weekday name             - Sunday to Saturday
     %w     day of week              - 1 (Monday) to 7 (Sunday)
     %E     day of month with suffix - 1st, 2nd, 3rd...
 Hour
     %H     hour                     - 00 to 23
     %k     hour                     - " 0" to "23"
     %i     hour                     - " 1" to "12"
     %I     hour                     - 01 to 12
     %p     AM or PM
 Minute, Second, Time zone
     %M     minute                   - 00 to 59
     %S     second                   - 00 to 59
     %Z     time zone                - "EDT"
     %z     time zone as GMT offset  - "+0100"
 Epoch (see NOTE 3 below)
     %s     seconds from 1/1/1970 GMT- negative if before 1/1/1970
     %o     seconds from Jan 1, 1970
            in the current time zone
 Date, Time
     %c     %a %b %e %H:%M:%S %Y     - Fri Apr 28 17:23:15 1995
     %C,%u  %a %b %e %H:%M:%S %z %Y  - Fri Apr 28 17:25:57 EDT 1995
     %g     %a, %d %b %Y %H:%M:%S %z - Fri, 28 Apr 1995 17:23:15 EDT
     %D     %m/%d/%y                 - 04/28/95
     %x     %m/%d/%y or %d/%m/%y     - 04/28/95 or 28/04/28
                                       (Depends on DateFormat variable)
     %l     date in ls(1) format (see NOTE 1 below)
              %b %e $H:$M            - Apr 28 17:23  (if within 6 months)
              %b %e  %Y              - Apr 28  1993  (otherwise)
     %r     %I:%M:%S %p              - 05:39:55 PM
     %R     %H:%M                    - 17:40
     %T,%X  %H:%M:%S                 - 17:40:58
     %V     %m%d%H%M%y               - 0428174095
     %Q     %Y%m%d                   - 19961025
     %q     %Y%m%d%H%M%S             - 19961025174058
     %P     %Y%m%d%H%M%S             - 1996102517:40:58
     %O     %Y-%m-%dT%H:%M:%S        - 1996-10-25T17:40:58
     %F     %A, %B %e, %Y            - Sunday, January  1, 1996
     %K     %Y-%j                    - 1997-045
 Special Year/Week formats (see NOTE 2 below)
     %G     year, Monday as first
            day of week              - 0001 to 9999
     %W     week of year, Monday
            as first day of week     - 01 to 53
     %L     year, Sunday as first
            day of week              - 0001 to 9999
     %U     week of year, Sunday
            as first day of week     - 01 to 53
     %J     %G-W%W-%w                - 1997-W02-2
 Other formats
     %n     insert a newline character
     %t     insert a tab character
     %%     insert a `%' character
     %+     insert a `+' character
 The following formats are currently unused but may be used in the future:
     N 1234567890 !@#$^&*()_|-=\`[];',./~{}:<>?
 They currently insert the character following the %, but may (and probably
 will) change in the future as new formats are added.

If a lone percent is the final character in a format, it is ignored.

The formats used in this routine were originally based on date.pl (version
3.2) by Terry McGonigal, as well as a couple taken from different versions
of the Solaris date(1) command.  Also, several have been added which are
unique to Date::Manip.

NOTE 1:

The ls format (%l) applies to date within the past OR future 6 months!

NOTE 2:

The %U, %W, %L, %G, and %J formats are used to support the ISO-8601 format:
YYYY-wWW-D.  In this format, a date is written as a year, the week of the
year, and the day of the week.  Technically, the week may be considered to
start on any day of the week, but Sunday and Monday are the both common
choices, so both are supported.

The %W and %G formats return the week-of-year and the year treating weeks
as starting on Monday.

The %U and %L formats return the week-of-year and the year treating weeks
as starting on Sunday.

Most of the time, the %L and %G formats returns the same value as the %Y
format, but there is a problem with days occurring in the first or last week
of the year.

The ISO-8601 representation of Jan 1, 1993 written in the YYYY-wWW-D format
is actually 1992-W53-5.  In other words, Jan 1 is treated as being in the
last week of the preceding year.  Depending on the year, days in the first
week of a year may belong to the previous year, and days in the final week
of a year may belong to the next year.  The week is assigned to the year
which has most of the days.  For example, if the week starts on Sunday,
then the last week of 2003 is 2003-12-28 to 2004-01-03.  This week is
assigned to 2003 since 4 of the days in it are in 2003 and only 3 of them
are in 2004.  The first week of 2004 starts on 2004-01-04.

The %U and %W formats return a week-of-year number from 01 to 53. %L and
%G return the corresponding year, and to get this type of information,
you should always use the (%W,%G) combination or (%U,%L) combination. %Y
should not be used as it will yield incorrect results.

%J returns the full ISO-8601 format (%G-W%W-%w).

NOTE 3:

The %s and %o formats return negative values if the date is before
the start of the epoch.  Other Unix utilities would return an error, or
a zero, so if you are going to use Date::Manip in conjunction with these,
be sure to check for a negative value.

=item B<ParseDateDelta>

 $delta = ParseDateDelta(\@args);
 $delta = ParseDateDelta($string);
 $delta = ParseDateDelta(\$string);

This takes an array and shifts a valid delta date (an amount of time)
from the array.  Recognized deltas are of the form:
  +Yy +Mm +Ww +Dd +Hh +MNmn +Ss
      examples:
         +4 hours +3mn -2second
         + 4 hr 3 minutes -2
         4 hour + 3 min -2 s
  +Y:+M:+W:+D:+H:+MN:+S
      examples:
         0:0:0:0:4:3:-2
         +4:3:-2
  mixed format
      examples:
         4 hour 3:-2

A field in the format +Yy is a sign, a number, and a string specifying
the type of field.  The sign is "+", "-", or absent (defaults to the
next larger element).  The valid strings specifying the field type
are:
   y:  y, yr, year, years
   m:  m, mon, month, months
   w:  w, wk, ws, wks, week, weeks
   d:  d, day, days
   h:  h, hr, hour, hours
   mn: mn, min, minute, minutes
   s:  s, sec, second, seconds

Also, the "s" string may be omitted.  The sign, number, and string may
all be separated from each other by any number of whitespace.

In the date, all fields must be given in the order: Y M W D H MN S.  Any
number of them may be omitted provided the rest remain in the correct
order.  In the 2nd (colon) format, from 2 to 7 of the fields may be given.
For example +D:+H:+MN:+S may be given to specify only four of the fields.
In any case, both the MN and S field may be present.  No spaces may be
present in the colon format.

Deltas may also be given as a combination of the two formats.  For example,
the following is valid: +Yy +D:+H:+MN:+S.  Again, all fields must be given
in the correct order.

The word "in" may be given (prepended in English) to the delta ("in 5 years")
and the word "ago" may be given (appended in English) ("6 months ago").  The
"in" is completely ignored.  The "ago" has the affect of reversing all signs
that appear in front of the components of the delta.  I.e. "-12 yr 6 mon ago"
is identical to "+12yr +6mon" (don't forget that there is an implied minus
sign in front of the 6 because when no sign is explicitly given, it carries
the previously entered sign).

One thing is worth noting.  The year/month and day/hour/min/sec parts are
returned in a "normalized" form.  That is, the signs are adjusted so as to
be all positive or all negative.  For example, "+ 2 day - 2hour" does not
return "0:0:0:2:-2:0:0".  It returns "+0:0:0:1:22:0:0" (1 day 22 hours
which is equivalent).  I find (and I think most others agree) that this is
a more useful form.

Since the year/month and day/hour/min/sec parts must be normalized
separately there is the possibility that the sign of the two parts will be
different.  So, the delta "+ 2years -10 months - 2 days + 2 hours" produces
the delta "+1:2:-0:1:22:0:0".

It is possible to include a sign for all elements that is output.  See the
configuration variable DeltaSigns below.

NOTE: The internal format of the delta changed in version 5.30 from
Y:M:D:H:MN:S to Y:M:W:D:H:MN:S .  Also, it is going to change again at some
point in the future to Y:M:W:D:H:MN:S*FLAGS .  Use the routine Delta_Format
to extract information rather than parsing it yourself.

=item B<Delta_Format>

 @str = Delta_Format($delta [,$mode], $dec,@format);
 $str = Delta_Format($delta [,$mode], $dec,@format);

This is similar to the UnixDate routine except that it extracts information
from a delta.  Unlike the UnixDate routine, most of the formats are 2
characters instead of 1.

Formats currently understood are:

   %Xv     : the value of the field named X
   %Xd     : the value of the field X, and all smaller fields, expressed in
             units of X
   %Xh     : the value of field X, and all larger fields, expressed in units
             of X
   %Xt     : the value of all fields expressed in units of X

   X is one of y,M,w,d,h,m,s (case sensitive).

   %%      : returns a "%"

So, the format "%hd" means the values of H, MN, and S expressed in hours.
So for the delta "0:0:0:0:2:30:0", this format returns 2.5.

Delta_Format can operate in two modes: exact and approximate. The exact
mode is done by default. Approximate mode can be done by passing in
the string "approx" as the 2nd argument.

In exact mode, Delta_Format only understands "exact" relationships. This
means that there can be no mixing of the Y/M and W/D/H/MN/S segments
because the relationship because, depending on when the delta occurs, there
is no exact relation between the number of years or months and the number
of days.

The two sections are treated completely separate from each other. So,
the delta "1:6:1:2:12:0:0" would return the following values:

  %yt = 1.5 (1 year, 6 months)
  %Mt = 18

  %dt = 9.5 (1 week, 2 days, 12 hours)

In approximate mode, the relationship of 1 year = 365.25 days is applied
(with 1 month equal to 1/12 of a year exactly). So the delta
"1:6:1:2:12:0:0" would return the following values:

  %dt = 557.375 (1.5 years of 365.25 days + 9.5 days)

If $dec is non-zero, the %Xd and %Xt values are formatted to contain $dec
decimal places.

=item B<ParseRecur>

 $recur = ParseRecur($string [,$base,$date0,$date1,$flags]);
 @dates = ParseRecur($string [,$base,$date0,$date1,$flags]);

A recurrence refers to a recurring event, and more specifically, an event
which occurs on a regular basis.  A fully specified recurring event
may requires up to four pieces of information.

First, it requires a description of the frequency of the event.  Examples
include "the first of every month", "every other day", "the 4th
Thursday of each month at 2:00 PM", and "every 2 hours and 30 minutes".

Second, it may require a base date to work from.  This piece of information
is not required for every type of recurrence.  For example, if the
frequency is "the first of every month", no base date is required.  All the
information about when the event occurs is included in the frequency
description.  If the frequency were "every other day" though, you need to
know at least one day on which the event occurred.

Third, the recurring event may have a range (a starting and ending date).

Fourth, there may be some flags included which modify the behavior of the
above information.

The fully specified recurrence is written as these 5 pieces of information
(both a start and end date) as an asterisk separated list:

  freq*flags*base*date0*date1

Here, base, date0, and date1 are any strings (which must not contain any
asterisks) which can be parsed by ParseDate.  flags is a comma separated
list of flags (described below), and freq is a string describing the
frequency of the recurring event.

The syntax of the frequency description is a colon separated list of the
format Y:M:W:D:H:MN:S (which stand for year, month, week, etc.).  One (and
only one) of the colons may optionally be replaced by an asterisk, or an
asterisk may be prepended to the string.  For example, the following are
all valid frequency descriptions:

  1:2:3:4:5:6:7
  1:2*3:4:5:6:7
 *1:2:3:4:5:6:7

But the following are NOT valid because they contain 2 or more asterisks:

  1:2*3:4:5*6:7
  1*2*3:4:5*6:7
 *1:2:3:4:5:6*7

If an asterisk is included, values to the left of it refer to the number of
times that time interval occurs between recurring events.  For example,
if the first part of the recurrence is:

  1:2*

this says that the recurring event occurs approximately every 1 year and 2
months.  I say approximately, because elements to the right of the asterisk,
as well as any flags included in the recurrence will affect when the actual
events occur.

If no asterisks are included, then the entire recurrence is of this form.
For example,

  0:0:0:1:12:0:0

refers to an event that occurs every 1 day, 12 hours.

Values that occur after an asterisk refer to a specific value for that type
of time element (i.e. exactly as it would appear on a calendar or a clock).
For example, if the recurrence ends with:

  *12:0:0

then the recurring event occurs at 12:00:00 (noon).

For example:

  0:0:2:1:0:0:0        every 2 weeks and 1 day
  0:0:0:0:5:30:0       every 5 hours and 30 minutes
  0:0:0:2*12:30:0      every 2 days at 12:30 (each day)

Values to the right of the asterisk can be listed a single values, ranges
(2 numbers separated by a dash "-"), or a comma separated list of values
or ranges.  In most cases, negative values are appropriate for the week
or day values. -1 stands for the last possible value, -2 for the second
to the last, etc.

Some examples are:

  0:0:0:1*2,4,6:0:0    every day at at 2:00, 4:00, and 6:00
  0:0:0:2*12-13:0,30:0 every other day at 12:00, 12:30, 13:00,
                       and 13:30
  0:1:0*-1:0:0:0       the last day of every month
  *1990-1995:12:0:1:0:0:0
                       Dec 1 in 1990 through 1995

There is no way to express the following with a single recurrence:

  every day at 12:30 and 1:00

You have to use two recurrences to do this.

When a non-zero day element occurs to the right of the asterisk, it can take
on multiple meanings, depending on the value of the month and week
elements.  It can refer to the day of the week, day of the month, or day of
the year.  Similarly, if a non-zero week element occurs to the right of
the asterisk, it actually refers to the nth time a certain day of the week
occurs, either in the month or in the year.

If the week element is non-zero and the day element is non-zero (and to the
right of the asterisk), the day element refers to the day of the week. It
can be any value from 1 to 7 (negative values -1 to -7 are also
allowed). If you use the ISO 8601 convention, the first day of the week is
Monday (though Date::Manip can use any day as the start of the week by
setting the FirstDay config variable).  So, assuming that you are using the
ISO 8601 convention, the following examples illustrate day-of-week
recurrences:

  0:1*4:2:0:0:0        4th Tuesday (day 2) of every month
  0:1*-1:2:0:0:0       last Tuesday of every month
  0:0:3*2:0:0:0        every 3rd Tuesday (every 3 weeks
                       on 2nd day of week)
  1:0*12:2:0:0:0       the 12th Tuesday of each year

If the week element is non-zero, and the day element is zero, the day
defaults to 1 (i.e. the first day of the week).

  0:1*2:0:0:0:0        the 2nd occurrence of FirstDay
                       in the year (typically Monday)
  0:1*2:1:0:0:0        the same

If the week element is zero and the month element is non-zero, the day
value is the day of the month (it can be from 1 to 31 or -1 to -31 counting
from the end of the month). If a value of 0 is given, it defaults to 1.

  3*1:0:2:12:0:0       every 3 years on Jan 2 at noon
  0:1*0:2:12,14:0:0    2nd of every month at 12:00 and 14:00
  0:1:0*-2:0:0:0       2nd to last day of every month

If the day given refers to the 29th, 30th, or 31st, in a month
that does not have that number of days, it is ignored. For example,
if you ask for the 31st of every month, it will return dates in Jan,
Mar, May, Jul, etc.  Months with fewer than 31 days will be ignored.

If both the month and week elements are zero, and the year element
is non-zero, the day value is the day of the year (1 to 365 or 366 -- or
the negative numbers to count backwards from the end of the year).

  1:0:0*45:0:0:0       45th day of every year

Specifying a day that doesn't occur in that year silently ignores that
year. The only result of this is that specifying +366 or -366 will ignore
all years except leap years.

I realize that this looks a bit cryptic, but after a discussion on the
CALENDAR mailing list, it appeared like there was no concise, flexible
notation for handling recurring events.  ISO 8601 notations were very bulky
and lacked the flexibility I wanted.  As a result, I developed this
notation (based on crontab formats, but with much more flexibility) which
fits in well with this module. Even better, it is able to express every
type of recurring event I could think of that is used in common life in
(what I believe to be) a very concise and elegant way.

If ParseRecur is called in scalar context, it returns a string containing a
fully specified recurrence (or as much of it as can be determined with
unspecified fields left blank).  In list context, it returns a list of all
dates referred to by a recurrence if enough information is given in the
recurrence.  All dates returned are in the range:

  date0 <= date < date1

The argument $string can contain any of the parts of a full recurrence.
For example:

  freq
  freq*flags
  freq**base*date0*date1

The only part which is required is the frequency description.  Any values
contained in $string are overridden or modified by values passed in as
parameters to ParseRecur.

NOTE: If a recurrence has a date0 and date1 in it AND a date0 and date1
are passed in to the function, both sets of criteria apply.  If flags are
passed in, they override any flags in the recurrence UNLESS the flags
passed in start with a plus (+) character in which case they are appended
to the flags in the recurrence.

NOTE: Base dates are only used with some types of recurrences.  For example,

  0:0:3*2:0:0:0        every 3rd Tuesday

requires a base date.  If a base date is specified which doesn't match the
criteria (for example, if a base date falling on Monday were passed in with
this recurrence), the base date is moved forward to the first relevant date.

Other dates do not require a base date.  For example:

  0:0*3:2:0:0:0        third Tuesday of every month

A recurrence written in the above format does NOT provide default values
for base, date0, or date1.  They must be specified in order to get a list
of dates.

A base date is not used entirely.  It is only used to provide the parts
necessary for the left part of a recurrence.  For example, the recurrence:

  1:3*0:4:0:0:0        every 1 year, 3 months on the 4th day of the month

would only use the year and month of the base date.

There are a small handful of English strings which can be parsed in place
of a numerical recur description.  These include:

  every 2nd day [in 1997]
  every 2nd day in June [1997]
  2nd day of every month [in 1997]
  2nd Tuesday of every month [in 1997]
  last Tuesday of every month [in 1997]
  every Tuesday [in 1997]
  every 2nd Tuesday [in 1997]
  every 2nd Tuesday in June [1997]

Each of these set base, date0, and date1 to a default value (the current
year with Jan 1 being the base date is the default if the year and month
are missing).

The following flags (case insensitive) are understood:

  PDn   : n is 1-7.  Means the previous day n not counting today
  PTn   : n is 1-7.  Means the previous day n counting today
  NDn   : n is 1-7.  Means the next day n not counting today
  NTn   : n is 1-7.  Means the next day n counting today

  FDn   : n is any number.  Means step forward n days.
  BDn   : n is any number.  Means step backward n days.
  FWn   : n is any number.  Means step forward n workdays.
  BWn   : n is any number.  Means step backward n workdays.

  CWD   : the closest work day (using the TomorrowFirst config variable).
  CWN   : the closest work day (looking forward first).
  CWP   : the closest work day (looking backward first).

  NWD   : next work day counting today
  PWD   : previous work day counting today
  DWD   : next/previous work day (TomorrowFirst config) counting today

  EASTER: select easter for this year (the M, W, D fields are ignored
          in the recur).

CWD, CWN, and CWP will usually return the same value, but if you are
starting at the middle day of a 3-day weekend (for example), it will return
either the first work day of the following week, or the last work day of
the previous week depending on whether it looks forward or backward first.

All flags are applied AFTER the recurrence dates are calculated, and they
may move a date outside of the date0 to date1 range.  No check is made for
this.

The workday flags do not act exactly the same as a business mode calculation.
For example, a date that is Saturday with a FW1 steps forward to the first
workday (i.e. Monday).

=item B<Date_Cmp>

 $flag = Date_Cmp($date1,$date2);

This takes two dates and compares them.  Almost all dates can be compared
using the Perl "cmp" command.  The only time this will not work is when
comparing dates in different time zones.  This routine will take that into
account.

NOTE:  This routine currently does little more than use "cmp", but once
the internal format for storing dates is in place (where time zone information
is kept as part of the date), this routine will become more important.  You
should use this routine in preparation for that version.

=item B<DateCalc>

 $d = DateCalc($d1,$d2 [,\$err] [,$mode]);

This takes two dates, deltas, or one of each and performs the appropriate
calculation with them.  Dates must be a string that can be parsed by
ParseDateString.  Deltas must be a string that can be parsed by
ParseDateDelta.  Two deltas add together to form a third delta.  A date
and a delta returns a 2nd date.  Two dates return a delta (the difference
between the two dates).

Since the two items can be interpreted as either dates or deltas, and since
many types of dates can be interpreted as deltas (and vice versa), it is
a good idea to pass the input through ParseDate or ParseDateDelta as
appropriate. For example, the string "09:00:00" can be interpreted either
as a date (today at 9:00:00) or a delta (9 hours). To avoid unexpected
results, avoid calling DateCalc as:

  $d = DateCalc("09:00:00",$someothervalue);

Instead, call it as:

  $d = DateCalc(ParseDate("09:00:00"),$someothervalue);

to force it to be a date, or:

  $d = DateCalc(ParseDateDelta("09:00:00"),$someothervalue);

to force it to be a delta. This will avoid unexpected results.

Note that in many cases, it is somewhat ambiguous what the delta
actually refers to.  Although it is ALWAYS known how many months in a
year, hours in a day, etc., it is NOT known (in the generals case) how
many days are in a month.  As a result, the part of the delta
containing month/year and the part with sec/min/hr/day must be treated
separately.  For example, "Mar 31, 12:00:00" plus a delta of 1month
2days would yield "May 2 12:00:00".  The year/month is first handled
while keeping the same date.  Mar 31 plus one month is Apr 31 (but
since Apr only has 30 days, it becomes Apr 30).  Apr 30 + 2 days is
May 2.  As a result, in the case where two dates are entered, the
resulting delta can take on two different forms.  By default
($mode=0), an absolutely correct delta (ignoring daylight saving time)
is returned in weeks, days, hours, minutes, and seconds.

If $mode is 1, the math is done using an approximate mode where a delta is
returned using years and months as well.  The year and month part is
calculated first followed by the rest.  For example, the two dates "Mar 12
1995" and "Apr 13 1995" would have an exact delta of "31 days" but in the
approximate mode, it would be returned as "1 month 1 day".  Also, "Mar 31"
and "Apr 30" would have deltas of "30 days" or "1 month" (since Apr 31
doesn't exist, it drops down to Apr 30).  Approximate mode is a more human
way of looking at things (you'd say 1 month and 2 days more often then 33
days), but it is less meaningful in terms of absolute time.  In approximate
mode $d1 and $d2 must be dates.  If either or both is a delta, the
calculation is done in exact mode.

If $mode is 2, a business mode is used.  That is, the calculation is done
using business days, ignoring holidays, weekends, etc.  In order to
correctly use this mode, a config file must exist which contains the
section defining holidays (see documentation on the config file below).
The config file can also define the work week and the hours of the work
day, so it is possible to have different config files for different
businesses.

For example, if a config file defines the workday as 08:00 to 18:00, a
work week consisting of Mon-Sat, and the standard (American) holidays, then
from Tuesday at 12:00 to the following Monday at 14:00 is 5 days and 2
hours.  If the "end" of the day is reached in a calculation, it
automatically switches to the next day.  So, Tuesday at 12:00 plus 6 hours
is Wednesday at 08:00 (provided Wed is not a holiday).  Also, a date that
is not during a workday automatically becomes the start of the next
workday.  So, Sunday 12:00 and Monday at 03:00 both automatically becomes
Monday at 08:00 (provided Monday is not a holiday).  In business mode, any
combination of date and delta may be entered, but a delta should not
contain a year or month field (weeks are fine though).

See Date::Manip::Calc for some additional comments about business mode calculations.

Note that a business week is treated the same as an exact week (i.e. from
Tuesday to Tuesday, regardless of holidays).  Because this means that the
relationship between days and weeks is NOT unambiguous, when a delta is
produced from two dates, it will be in terms of d/h/mn/s (i.e. no week
field).

If $mode is 3 (which only applies when two dates are passed in), an exact
business mode is used.  In this case, it returns a delta as an exact number
of business days/hours/etc. between the two.  Weeks, months, and years are
ignored.

Any other non-nil value of $mode is treated as $mode=1 (approximate mode).

The mode can be automatically set in the dates/deltas passed by including a
key word somewhere in it.  For example, in English, if the word
"approximately" is found in either of the date/delta arguments, approximate
mode is forced.  Likewise, if the word "business" or "exactly" appears,
business/exact mode is forced (and $mode is ignored).  So, the two
following are equivalent:

   $date = DateCalc("today","+ 2 business days",\$err);
   $date = DateCalc("today","+ 2 days",\$err,2);

Note that if the keyword method is used instead of passing in $mode, it is
important that the keyword actually appear in the argument passed in to
DateCalc.  The following will NOT work:

   $delta = ParseDateDelta("+ 2 business days");
   $today = ParseDate("today");
   $date = DateCalc($today,$delta,\$err);

because the mode keyword is removed from a date/delta by the parse routines,
and the mode is reset each time a parse routine is called.  Since DateCalc
parses both of its arguments, whatever mode was previously set is ignored.

If \$err is passed in, it is set to:
   1 is returned if $d1 is not a delta or date
   2 is returned if $d2 is not a delta or date
   3 is returned if the date is outside the years 1000 to 9999
This argument is optional, but if included, it must come before $mode.

Nothing is returned if an error occurs.

When a delta is returned, the signs such that it is strictly positive or
strictly negative ("1 day - 2 hours" would never be returned for example).
The only time when this cannot be enforced is when two deltas with a
year/month component are entered.  In this case, only the signs on the
day/hour/min/sec part are standardized.

=item B<Date_SetTime>

 $date = Date_SetTime($date,$hr,$min,$sec);
 $date = Date_SetTime($date,$time);

This takes a date (any string that may be parsed by ParseDateString) and
sets the time in that date.  For example, one way to get the time for 7:30
tomorrow would be to use the lines:

   $date = ParseDate("tomorrow");
   $date = Date_SetTime($date,"7:30");

Note that in this routine (as well as the other routines below which use
a time argument), no real parsing is done on the times.  As a result,

   $date = Date_SetTime($date,"13:30");

works, but

   $date = Date_SetTime($date,"1:30 PM");

doesn't.

=item B<Date_SetDateField>

 $date = Date_SetDateField($date,$field,$val [,$nocheck]);

This takes a date and sets one of its fields to a new value.  $field is
any of the strings "y", "m", "d", "h", "mn", "s" (case insensitive) and
$val is the new value.

If $nocheck is non-zero, no check is made as to the validity of the date.

=item B<Date_GetPrev>

 $date = Date_GetPrev($date,$dow, $curr [,$hr,$min,$sec]);
 $date = Date_GetPrev($date,$dow, $curr [,$time]);
 $date = Date_GetPrev($date,undef,$curr,$hr,$min,$sec);
 $date = Date_GetPrev($date,undef,$curr,$time);

This takes a date (any string that may be parsed by ParseDateString) and finds
the previous occurrence of either a day of the week, or a certain time of day.

If $dow is defined, the previous occurrence of the day of week is returned.
$dow may either be a string (such as "Fri" or "Friday") or a number
(between 1 and 7).  The date of the previous $dow is returned.

If $date falls on the day of week given by $dow, the date returned depends
on $curr.  If $curr is 0, the date returned is a week before $date.  If
$curr is 1, the date returned is the same as $date.  If $curr is 2, the date
returned (including the time information) is required to be before $date.

If a time is passed in (either as separate hours, minutes, seconds or as a
time in HH:MM:SS or HH:MM format), the time on this date is set to it.  The
following examples should illustrate the use of Date_GetPrev:

    date                   dow    curr  time            returns
    Fri Nov 22 18:15:00    Thu    any   12:30           Thu Nov 21 12:30:00
    Fri Nov 22 18:15:00    Fri    0     12:30           Fri Nov 15 12:30:00
    Fri Nov 22 18:15:00    Fri    1/2   12:30           Fri Nov 22 12:30:00

    Fri Nov 22 18:15:00    Fri    1     18:30           Fri Nov 22 18:30:00
    Fri Nov 22 18:15:00    Fri    2     18:30           Fri Nov 15 18:30:00

If $dow is undefined, then a time must be entered, and the date returned is
the previous occurrence of this time.  If $curr is non-zero, the current
time is returned if it matches the criteria passed in.  In other words, the
time returned is the last time that a digital clock (in 24 hour mode) would
have displayed the time you passed in.  If you define hours, minutes and
seconds default to 0 and you might jump back as much as an entire day.  If
hours are undefined, you are looking for the last time the minutes/seconds
appeared on the digital clock, so at most, the time will jump back one hour.

    date               curr  hr     min    sec      returns
    Nov 22 18:15:00    0/1   18     undef  undef    Nov 22 18:00:00
    Nov 22 18:15:00    0/1   18     30     0        Nov 21 18:30:00
    Nov 22 18:15:00    0     18     15     undef    Nov 21 18:15:00
    Nov 22 18:15:00    1     18     15     undef    Nov 22 18:15:00
    Nov 22 18:15:00    0     undef  15     undef    Nov 22 17:15:00
    Nov 22 18:15:00    1     undef  15     undef    Nov 22 18:15:00

=item B<Date_GetNext>

 $date = Date_GetNext($date,$dow, $curr [,$hr,$min,$sec]);
 $date = Date_GetNext($date,$dow, $curr [,$time]);
 $date = Date_GetNext($date,undef,$curr,$hr,$min,$sec);
 $date = Date_GetNext($date,undef,$curr,$time);

Similar to Date_GetPrev.

=item B<Date_IsHoliday>

 $name = Date_IsHoliday($date);

This returns undef if $date is not a holiday, or a string containing the
name of the holiday otherwise.  An empty string is returned for an unnamed
holiday.

=item B<Events_List>

 $ref = Events_List($date);
 $ref = Events_List($date ,0      [,$flag]);
 $ref = Events_List($date0,$date1 [,$flag]);

This returns a list of events.  Events are defined in the Events section
of the config file (discussed below).

In the first form (a single argument), $date is any string containing a
date.  A list of events active at that precise time will be returned.
The format is similar to when $flag=0, except only a single time will
be returned.

In all other cases, a range of times will be used.  If the 2nd argument
evaluates to 0, the range of times will be the 24 hour period from
midnight to midnight containing $date.  Otherwise, the range is given
by the two dates.

The value of $flag determines the format of the information that is
returned.

With $flag=0, the events are returned as a reference to a list of the form:

  [ date, [ list_of_events ], date, [ list_of_events ], ... ]

For example, if the following events are defined (using the syntax
discussed below in the description of the Event section of the config
file):

  2000-01-01 ; 2000-03-21  = Winter
  2000-03-22 ; 2000-06-21  = Spring
  2000-02-01               = Event1
  2000-05-01               = Event2
  2000-04-01-12:00:00      = Event3

might result in the following output:

  Events_List("2000-04-01")
   => [ 2000040100:00:00, [ Spring ] ]

  Events_List("2000-04-01 12:30");
   => [ 2000040112:30:00, [ Spring, Event3 ] ]

  Events_List("2000-04-01",0);
   => [ 2000040100:00:00, [ Spring ],
        2000040112:00:00, [ Spring, Event3 ],
        2000040113:00:00, [ Spring ] ]

  Events_List("2000-03-15","2000-04-10");
   => [ 2000031500:00:00, [ Winter ],
        2000032200:00:00, [ Spring ]
        2000040112:00:00, [ Spring, Event3 ]
        2000040113:00:00, [ Spring ] ]

Much more complicated events can be defined using recurrences.

When $flag is non-zero, the format of the output is changed.  If $flag
is 1, then a tally of the amount of time given to each event is returned.
Time for which two or more events apply is counted for both.

  Events_List("2000-03-15","2000-04-10",1);
   => { Winter => +0:0:1:0:0:0:0,
        Spring => +0:0:2:5:0:0:0,
        Event3 => +0:0:0:0:1:0:0 }

When $flag is 2, a more complex tally with no event counted twice is
returned.

  Events_List("2000-03-15","2000-04-10",2);
   => { Winter => +0:0:1:0:0:0:0,
        Spring => +0:0:2:4:23:0:0,
        Event3+Spring => +0:0:0:0:1:0:0 }

The hash contains one element for each combination of events.

=item B<Date_DayOfWeek>

 $day = Date_DayOfWeek($m,$d,$y);

Returns the day of the week (1 for Monday, 7 for Sunday).

All arguments must be numeric.

=item B<Date_SecsSince1970>

 $secs = Date_SecsSince1970($m,$d,$y,$h,$mn,$s);

Returns the number of seconds since Jan 1, 1970 00:00 (negative if date is
earlier).

All arguments must be numeric.

=item B<Date_SecsSince1970GMT>

 $secs = Date_SecsSince1970GMT($m,$d,$y,$h,$mn,$s);

Returns the number of seconds since Jan 1, 1970 00:00 GMT (negative if date
is earlier).  If CurrTZ is "IGNORE", the number will be identical to
Date_SecsSince1970 (i.e. the date given will be treated as being in GMT).

All arguments must be numeric.

=item B<Date_DaysSince1BC>

 $days = Date_DaysSince1BC($m,$d,$y);

Returns the number of days since Dec 31, 1BC.  This includes the year 0000.

All arguments must be numeric.

=item B<Date_DayOfYear>

 $day = Date_DayOfYear($m,$d,$y);

Returns the day of the year (001 to 366)

All arguments must be numeric.

=item B<Date_NthDayOfYear>

 ($y,$m,$d,$h,$mn,$s) = Date_NthDayOfYear($y,$n);

Returns the year, month, day, hour, minutes, and decimal seconds given
a floating point day of the year.

All arguments must be numeric.  $n must be greater than or equal to 1
and less than 366 on non-leap years and 367 on leap years.

NOTE: When $n is a decimal number, the results are non-intuitive perhaps.
Day 1 is Jan 01 00:00.  Day 2 is Jan 02 00:00.  Intuitively, you
might think of day 1.5 as being 1.5 days after Jan 01 00:00, but this
would mean that Day 1.5 was Jan 02 12:00 (which is later than Day 2).
The best way to think of this function is a time line starting at 1 and
ending at 366 (in a non-leap year).  In terms of a delta, think of $n
as the number of days after Dec 31 00:00 of the previous year.

=item B<Date_DaysInYear>

 $days = Date_DaysInYear($y);

Returns the number of days in the year (365 or 366)

=item B<Date_DaysInMonth>

 $days = Date_DaysInMonth($m,$y);

Returns the number of days in the month.

=item B<Date_WeekOfYear>

 $wkno = Date_WeekOfYear($m,$d,$y,$first);

Figure out week number.  $first is the first day of the week which is
usually 1 (Monday) or 7 (Sunday), but could be any number between 1 and 7
in practice.

All arguments must be numeric.

NOTE: This routine should only be called in rare cases.  Use UnixDate with
the %W, %U, %J, %L formats instead.  This routine returns a week between 0
and 53 which must then be "fixed" to get into the ISO-8601 weeks from 1 to
53.  A date which returns a week of 0 actually belongs to the last week of
the previous year.  A date which returns a week of 53 may belong to the
first week of the next year.

=item B<Date_LeapYear>

 $flag = Date_LeapYear($y);

Returns 1 if the argument is a leap year
Written by David Muir Sharnoff <muir@idiom.com>

=item B<Date_DaySuffix>

 $day = Date_DaySuffix($d);

Add `st', `nd', `rd', `th' to a date (i.e. 1st, 22nd, 29th).  Works for
international dates.

=item B<Date_TimeZone>

 $tz = Date_TimeZone;

This determines and returns the local time zone.  If it is unable to determine
the local time zone, the following error occurs:

   ERROR: Date::Manip unable to determine Time Zone.

See The TIME ZONES section below for more information.

=item B<Date_ConvTZ>

 $date = Date_ConvTZ($date);
 $date = Date_ConvTZ($date,$from);
 $date = Date_ConvTZ($date,"",$to [,$errlev]);
 $date = Date_ConvTZ($date,$from,$to [,$errlev]);

This converts a date (which MUST be in the format returned by ParseDate)
from one time zone to another.

If it is called with no arguments, the date is converted from the local
time zone to the time zone specified by the config variable ConvTZ (see
documentation on ConvTZ below).  If ConvTZ is set to "IGNORE", no
conversion is done.

If called with $from but no $to, the time zone is converted from the
time zone in $from to ConvTZ (of TZ if ConvTZ is not set).  Again, no
conversion is done if ConvTZ is set to "IGNORE".

If called with $to but no $from, $from defaults to ConvTZ (if set) or the
local time zone otherwise.  Although this does not seem immediately obvious,
it actually makes sense.  By default, all dates that are parsed are
converted to ConvTZ, so most of the dates being worked with will be stored
in that time zone.

If Date_ConvTZ is called with both $from and $to, the date is converted
from the time zone $from to $to.

NOTE: As in all other cases, the $date returned from Date_ConvTZ has no
time zone information included as part of it, so calling UnixDate with the
"%z" format will return the time zone that Date::Manip is working in
(usually the local time zone).

Example:  To convert 2/2/96 noon PST to CST (regardless of what time zone
you are in, do the following:

 $date = ParseDate("2/2/96 noon");
 $date = Date_ConvTZ($date,"PST","CST");

Both time zones MUST be in one of the formats listed below in the section
TIME ZONES.

If an error occurs, $errlev determines what happens:

  0   : the program dies
  1   : a warning is produced and nothing is returned
  2   : the function silently returns nothing

=item B<Date_IsWorkDay>

  $flag = Date_IsWorkDay($date [,$flag]);

This returns 1 if $date is a work day.  If $flag is non-zero, the time is
checked to see if it falls within work hours.  It returns an empty string
if $date is not valid.

=item B<Date_NextWorkDay>

  $date = Date_NextWorkDay($date,$off [,$flag]);

Finds the day $off work days from now.  If $flag is non-zero, we must also
take into account the time of day.

If $flag is zero, day 0 is today (if today is a workday) or the
next work day if it isn't.  In any case, the time of day is unaffected.

If $flag is non-zero, day 0 is now (if now is part of a workday) or the
start of the very next work day.

=item B<Date_PrevWorkDay>

  $date = Date_PrevWorkDay($date,$off [,$flag]);

Similar to Date_NextWorkDay.

=item B<Date_NearestWorkDay>

  $date = Date_NearestWorkDay($date [,$tomorrowfirst]);

This looks for the work day nearest to $date.  If $date is a work day, it
is returned.  Otherwise, it will look forward or backwards in time 1 day
at a time until a work day is found.  If $tomorrowfirst is non-zero (or if
it is omitted and the config variable TomorrowFirst is non-zero), we look
to the future first.  Otherwise, we look in the past first.  In other words,
in a normal week, if $date is Wednesday, $date is returned.  If $date is
Saturday, Friday is returned.  If $date is Sunday, Monday is returned.  If
Wednesday is a holiday, Thursday is returned if $tomorrowfirst is non-nil
or Tuesday otherwise.

=item B<DateManipVersion>

  $version = DateManipVersion;

Returns the version of Date::Manip.

=back

=head1 TIME ZONES

With the release of Date::Manip 6.00, time zones and daylight saving
time are now fully supported in Date::Manip. 6.00 uses information from
several standards (most importantly the Olson zoneinfo database) to get
a list of all known time zones.

Unfortunately, 6.00 requires a newer version of perl, so I will continue
to support the 5.xx release for a while. However, the way I will support
time zones in 5.xx has changed. Previously, new time zones would be added
on request. That is no longer the case. Time zones for 5.xx are now generated
automatically from those available in 6.00.

The following time zone names are currently understood (and can be used in
parsing dates).  These are zones defined in RFC 822.

    Universal:  GMT, UT
    US zones :  EST, EDT, CST, CDT, MST, MDT, PST, PDT
    Military :  A to Z (except J)
    Other    :  +HHMM or -HHMM
    ISO 8601 :  +HH:MM, +HH, -HH:MM, -HH

In addition, the following time zone abbreviations are also accepted. These
do not come from a standard, but were included in previous releases of
Date::Manip 5.xx and are preserved here for backward compatibility:

   IDLW    -1200    International Date Line West
   NT      -1100    Nome
   SAT     -0400    Chile
   CLDT    -0300    Chile Daylight
   AT      -0200    Azores
   MEWT    +0100    Middle European Winter
   MEZ     +0100    Middle European
   FWT     +0100    French Winter
   GB      +0100    GMT with daylight savings
   SWT     +0100    Swedish Winter
   MESZ    +0200    Middle European Summer
   FST     +0200    French Summer
   METDST  +0200    An alias for MEST used by HP-UX
   EETDST  +0300    An alias for eest used by HP-UX
   EETEDT  +0300    Eastern Europe, USSR Zone 1
   BT      +0300    Baghdad, USSR Zone 2
   IT      +0330    Iran
   ZP4     +0400    USSR Zone 3
   ZP5     +0500    USSR Zone 4
   IST     +0530    Indian Standard
   ZP6     +0600    USSR Zone 5
   AWST    +0800    Australian Western Standard
   ROK     +0900    Republic of Korea
   AEST    +1000    Australian Eastern Standard
   ACDT    +1030    Australian Central Daylight
   CADT    +1030    Central Australian Daylight
   AEDT    +1100    Australian Eastern Daylight
   EADT    +1100    Eastern Australian Daylight
   NZT     +1200    New Zealand
   IDLE    +1200    International Date Line East

All other time zone abbreviations come from the standards. In many
cases, an abbreviation may be used for multiple time zones. For
example, NST stands for Newfoundland Standard -0330 and North Sumatra
+0630.  In these cases, only 1 of the two is available. I have tried
to use the most recent definition, and of those (if multiple time zones
use the abbreviation), the most commonly used. I don't claim that I'm
correct in all cases, but I've done the best I could.

The list of abbreviations available is documented in the
Date::Manip::DM5abbrevs document.

Date::Manip must be able to determine the time zone the user is in.  It does
this by looking in the following places:

   $Date::Manip::TZ (set with Date_Init or in Manip.pm)
   $ENV{TZ}
   the Unix `date` command (if available)
   $main::TZ
   /etc/TIMEZONE
   /etc/time zone

At least one of these should contain a time zone in one of the supported
forms.  If none do by default, the TZ variable must be set with Date_Init.

The time zone may be in the STD#DST format (in which case both abbreviations
must be in the table above) or any of the formats described above.  The
STD#DST format is NOT available when parsing a date however.  The following
forms are also available and are treated similar to the STD#DST forms:

      US/Pacific
      US/Mountain
      US/Central
      US/Eastern
      Canada/Pacific
      Canada/Mountain
      Canada/Central
      Canada/Eastern

=head1 CUSTOMIZING DATE::MANIP

There are a number of variables which can be used to customize the way
Date::Manip behaves.  There are also several ways to set these variables.

At the top of the Manip.pm file, there is a section which contains all
customization variables.  These provide the default values.

These can be overridden in a global config file if one is present (this
file is optional).  If the GlobalCnf variable is set in the Manip.pm file,
it contains the full path to a config file.  If the file exists, its
values will override those set in the Manip.pm file.  A sample config file
is included with the Date::Manip distribution.  Modify it as appropriate
and copy it to some appropriate directory and set the GlobalCnf variable in
the Manip.pm file.

Each user can have a personal config file which is of the same form as the
global config file.  The variables PersonalCnf and PersonalCnfPath set the
name and search path for the personal config file.  This file is also
optional.  If present, it overrides any values set in the global file.

NOTE: if you use business mode calculations, you must have a config file
(either global or personal) since this is the only place where you can
define holidays.

Finally, any variables passed in through Date_Init override all other
values.

A config file can be composed of several sections.  The first section sets
configuration variables.  Lines in this section are of the form:

   VARIABLE = VALUE

For example, to make the default language French, include the line:

   Language = French

Only variables described below may be used.  Blank lines and lines beginning
with a pound sign (#) are ignored.  All spaces are optional and strings are
case insensitive.

A line which starts with an asterisk (*) designates a new section.  For
example, the HOLIDAY section starts with a line:

   *Holiday

The various sections are defined below.

=head1 DATE::MANIP VARIABLES

All Date::Manip variables which can be used are described in the following
section.

=over 4

=item B<IgnoreGlobalCnf>

If this variable is used (any value is ignored), the global config file
is not read.  It must be present in the initial call to Date_Init or the
global config file will be read.

=item B<EraseHolidays>

If this variable is used (any value is ignored), the current list of
defined holidays is erased.  A new set will be set the next time a
config file is read in.  This can be set in either the global config file
or as a Date_Init argument (in which case holidays can be read in from
both the global and personal config files) or in the personal config file
(in which case, only holidays in the personal config file are counted).

=item B<PathSep>

This is a regular expression used to separate multiple paths.  For example,
on Unix, it defaults to a colon (:) so that multiple paths can be written
PATH1:PATH2 .  For Win32 platforms, it defaults to a semicolon (;) so that
paths such as "c:\;d:\" will work.

=item B<GlobalCnf>

This variable can be passed into Date_Init to point to a global
configuration file.  The value must be the complete path to a config file.

By default, no global config file is read.  Any time a global config file
is read, the holidays are erased.

Paths may have a tilde (~) expansion on platforms where this is supported
(currently Unix and VMS).

=item B<PersonalCnf>

This variable can be passed into Date_Init or set in a global config file
to set the name of the personal configuration file.

The default name for the config file is .DateManip.cnf on all Unix
platforms and Manip.cnf on all non-Unix platforms (because some of them
insist on 8.3 character filenames :-).

=item B<PersonalCnfPath>

This is a list of paths separated by the separator specified by the PathSep
variable.  These paths are each checked for the PersonalCnf config file.

Paths may have a tilde (~) expansion on platforms where this is supported
(currently Unix and VMS).

=item B<Language>

Date::Manip can be used to parse dates in many different languages.
Currently, it is configured to read  the following languages (the version
in which they added is included for historical interest):

  English      (default)
  French       (5.02)
  Swedish      (5.05)
  German       (5.31)
  Dutch        (5.32)     aka Nederlands
  Polish       (5.32)
  Spanish      (5.33)
  Portuguese   (5.34)
  Romanian     (5.35)
  Italian      (5.35)
  Russian      (5.41)
  Turkish      (5.41)
  Danish       (5.41)

Others can be added easily.  Language is set to the language used to parse
dates.  If you are interested in providing a translation for a new
language, email me (see the AUTHOR section below) and I'll send you a list
of things that I need.

=item B<DateFormat>

Different countries look at the date 12/10 as Dec 10 or Oct 12.  In the
United States, the first is most common, but this certainly doesn't hold
true for other countries.  Setting DateFormat to "US" forces the first
behavior (Dec 10).  Setting DateFormat to anything else forces the second
behavior (Oct 12).

=item B<TZ>

If set, this defines the local time zone.  See the TIME ZONES section above
for information on its format.

=item B<ConvTZ>

All date comparisons and calculations must be done in a single time zone in
order for them to work correctly.  So, when a date is parsed, it should be
converted to a specific time zone.  This allows dates to easily be compared
and manipulated as if they are all in a single time zone.

The ConvTZ variable determines which time zone should be used to store dates
in.  If it is left blank, all dates are converted to the local time zone
(see the TZ variable above).  If it is set to one of the time zones listed
above, all dates are converted to this time zone.  Finally, if it is set to
the string "IGNORE", all time zone information is ignored as the dates are
read in (in this case, the two dates "1/1/96 12:00 GMT" and "1/1/96 12:00
EST" would be treated as identical).

=item B<Internal>

When a date is parsed using ParseDate, that date is stored in an internal
format which is understood by the Date::Manip routines UnixDate and
DateCalc.  Originally, the format used to store the date internally was:

   YYYYMMDDHH:MN:SS

It has been suggested that I remove the colons (:) to shorten this to:

   YYYYMMDDHHMNSS

The main advantage of this is that some databases are colon delimited which
makes storing a date from Date::Manip tedious.

In order to maintain backwards compatibility, the Internal variable was
introduced.  Set it to 0 (to use the old format) or 1 (to use the new
format).

=item B<FirstDay>

It is sometimes necessary to know what day of week is regarded as first.
By default, this is set to Monday, but many countries and people will
prefer Sunday (and in a few cases, a different day may be desired).  Set
the FirstDay variable to be the first day of the week (1=Monday, 7=Sunday)
Monday should be chosen to to comply with ISO 8601.

=item B<WorkWeekBeg, WorkWeekEnd>

The first and last days of the work week.  By default, Monday and Friday.
WorkWeekBeg must come before WorkWeekEnd numerically.  The days are
numbered from 1 (Monday) to 7 (Sunday).

There is no way to handle an odd work week of Thu to Mon for example or 10
days on, 4 days off.

=item B<WorkDay24Hr>

If this is non-nil, a work day is treated as being 24 hours long.  The
WorkDayBeg and WorkDayEnd variables are ignored in this case.

=item B<WorkDayBeg, WorkDayEnd>

The times when the work day starts and ends.  WorkDayBeg must come before
WorkDayEnd (i.e. there is no way to handle the night shift where the work
day starts one day and ends another).  Also, the workday MUST be more than
one hour long (of course, if this isn't the case, let me know... I want a
job there!).

The time in both can be in any valid time format (including international
formats), but seconds will be ignored.

=item B<TomorrowFirst>

Periodically, if a day is not a business day, we need to find the nearest
business day to it.  By default, we'll look to "tomorrow" first, but if this
variable is set to 0, we'll look to "yesterday" first.  This is only used in
the Date_NearestWorkDay and is easily overridden (see documentation for that
function).

=item B<DeltaSigns>

Prior to Date::Manip version 5.07, a negative delta would put negative
signs in front of every component (i.e. "0:0:-1:-3:0:-4").  By default,
5.07 changes this behavior to print only 1 or two signs in front of the
year and day elements (even if these elements might be zero) and the sign
for year/month and day/hour/minute/second are the same.  Setting this
variable to non-zero forces deltas to be stored with a sign in front of
every element (including elements equal to 0).

=item B<Jan1Week1>

ISO 8601 states that the first week of the year is the one which contains
Jan 4 (i.e. it is the first week in which most of the days in that week
fall in that year).  This means that the first 3 days of the year may
be treated as belonging to the last week of the previous year.  If this
is set to non-nil, the ISO 8601 standard will be ignored and the first
week of the year contains Jan 1.

=item B<YYtoYYYY>

By default, a 2 digit year is treated as falling in the 100 year period of
CURR-89 to CURR+10.  YYtoYYYY may be set to any integer N to force a 2
digit year into the period CURR-N to CURR+(99-N).  A value of 0 forces
the year to be the current year or later.  A value of 99 forces the year
to be the current year or earlier.  Since I do no checking on the value of
YYtoYYYY, you can actually have it any positive or negative value to force
it into any century you want.

YYtoYYYY can also be set to "C" to force it into the current century, or
to "C##" to force it into a specific century.  So, in 1998, "C" forces
2 digit years to be 1900-1999 and "C18" would force it to be 1800-1899.

It can also be set to the form "C####" to force it into a specific 100
year period.  C1950 refers to 1950-2049.

=item B<UpdateCurrTZ>

If a script is running over a long period of time, the time zone may change
during the course of running it (i.e. when daylight saving time starts or
ends).  As a result, parsing dates may start putting them in the wrong time
zone.  Since a lot of overhead can be saved if we don't have to check the
current time zone every time a date is parsed, by default checking is turned
off.  Setting this to non-nil will force time zone checking to be done every
time a date is parsed... but this will result in a considerable performance
penalty.

A better solution would be to restart the process on the two days per year
where the time zone switch occurs.

=item B<IntCharSet>

If set to 0, use the US character set (7-bit ASCII) to return strings such
as the month name.  If set to 1, use the appropriate international character
set.  For example, If you want your French representation of December to
have the accent over the first "e", you'll want to set this to 1.

=item B<ForceDate>

This variable can be set to a date in the format: YYYY-MM-DD-HH:MN:SS
to force the current date to be interpreted as this date.  Since the current
date is used in parsing, this string will not be parsed and MUST be in the
format given above.

=item B<TodayIsMidnight>

If set to a true value (e.g. 1), then "today" will mean the same as
"midnight today"; otherwise it will mean the same as "now".

=back

=head1 HOLIDAY SECTION

The holiday section of the config file is used to define holidays.  Each
line is of the form:

   DATE = HOLIDAY

HOLIDAY is the name of the holiday (or it can be blank in which case the
day will still be treated as a holiday... for example the day after
Thanksgiving or Christmas is often a work holiday though neither are
named).

DATE is a string which can be parsed to give a valid date in any year.  It
can be of the form

   Date
   Date + Delta
   Date - Delta
   Recur

A valid holiday section would be:

   *Holiday

   1/1                             = New Year's Day
   third Monday in Feb             = Presidents' Day
   fourth Thu in Nov               = Thanksgiving

   # The Friday after Thanksgiving is an unnamed holiday most places
   fourth Thu in Nov + 1 day       =

   1*0:0:0:0:0:0*EASTER            = Easter
   1*11:0:11:0:0:0*DWD             = Veteran's Day (observed)
   1*0:0:0:0:0:0*EASTER,PD5        = Good Friday

In a Date + Delta or Date - Delta string, you can use business mode by
including the appropriate string (see documentation on DateCalc) in the
Date or Delta.  So (in English), the first workday before Christmas could
be defined as:

   12/25 - 1 business day          =

The dates may optionally contain the year.  For example, the dates

  1/1
  1/1/1999

refers to Jan 1 in any year or in only 1999 respectively.  For dates that
refer to any year, the date must be written such that by simply appending
the year (separated by spaces) it can be correctly interpreted.  This
will work for everything except ISO 8601 dates, so ISO 8601 dates may
not be used in this case.

Note that the dates are specified in whatever format is set using the
Date_Init options, so if the standard parsing is D/M/YYYY, you would
need to specify it as:

   25/12/2002           = Christmas

In cases where you are interested in business type calculations, you'll
want to define most holidays using recurrences, since they can define
when a holiday is celebrated in the financial world.  For example,
Christmas should be defined as:

   1*12:0:24:0:0:0*FW1  = Christmas

NOTE: It was pointed out to me that using a similar type recurrence to
define New Years does not work.  The recurrence:

   1*12:0:31:0:0:0*FW1

fails (worse, it goes into an infinite loop).  The problem is that each
holiday definition is applied to a specific year and it expects to find
the holiday for that year.  When this recurrence is applied to the year
1995, it returns the holiday for 1996 and fails.

Use the recurrence:

   1*1:0:1:0:0:0*NWD

instead.

If you wanted to define both Christmas and Boxing days (Boxing is the
day after Christmas, and is celebrated in some parts of the world), you
could do it in one of the following ways:

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:25:0:0:0*FW1  = Boxing

    1*12:0:24:0:0:0*FW1 = Christmas
   01*12:0:24:0:0:0*FW1 = Boxing

   1*12:0:24:0:0:0*FW1   = Christmas
   1*12:0:25:0:0:0*FW1,a = Boxing

The following examples will NOT work:

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:24:0:0:0*FW2  = Boxing

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:24:0:0:0*FW1  = Boxing

The reasoning behind all this is as follows:

Holidays go into affect the minute they are parsed.  So, in the case of:

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:24:0:0:0*FW2  = Boxing

the minute the first line is parsed, Christmas is defined as a holiday.
The second line then steps forward 2 work days (skipping Christmas since
that's no longer a work day) and define the work day two days after
Christmas, NOT the day after Christmas.

An good alternative would appear to be:

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:24:0:0:0*FW1  = Boxing

This unfortunately fails because the recurrences are currently stored in a
hash.  Since these two recurrences are identical, they fail (the first one
is overwritten by the second and in essence, Christmas is never defined).

To fix this, make them unique with either a fake flag (which is ignored):

   1*12:0:24:0:0:0*FW1,a  = Boxing

or adding an innocuous 0 somewhere:

   01*12:0:24:0:0:0*FW1   = Boxing

The other good alternative would be to make two completely different
recurrences such as:

   1*12:0:24:0:0:0*FW1  = Christmas
   1*12:0:25:0:0:0*FW1  = Boxing

At times, you may want to switch back and forth between two holiday files.
This can be done by calling the following:

  Date_Init("EraseHolidays=1","PersonalCnf=FILE1");
  ...
  Date_Init("EraseHolidays=1","PersonalCnf=FILE2");
  ...

=head1 EVENTS SECTION

The Events section of the config file is similar to the Holiday section.
It is used to name certain days or times, but there are a few important
differences:

=over 4

=item B<Events can be assigned to any time and duration>

All holidays are exactly 1 day long.  They are assigned to a period
of time from midnight to midnight.

Events can be based at any time of the day, and may be of any duration.

=item B<Events don't affect business mode calculations>

Unlike holidays, events are completely ignored when doing business
mode calculations.

=back

Whereas holidays were added with business mode math in mind, events
were added with calendar and scheduling applications in mind.

Every line in the events section is of the form:

   EVENT = NAME

where NAME is the name of the event, and EVENT defines when it occurs
and its duration.  An EVENT can be defined in the following ways:

   Date
   Date*

   Date  ; Date
   Date  ; Delta

Here, Date* refers to a string containing a Date with NO TIME fields
(Jan 12, 1/1/2000, 2010-01-01) while Date does contain time fields.
Similarly, Recur* stands for a recurrence with the time fields all
equal to 0) while Recur stands for a recurrence with at least one
non-zero time field.

Both Date* and Recur* refer to an event very similar to a holiday which
goes from midnight to midnight.

Date and Recur refer to events which occur at the time given and with
a duration of 1 hour.

Events given by "Date ; Date", "Date ; Delta", and "Recur ; Delta"
contain both the starting date and either ending date or duration.

Events given as three elements "Date ; Delta ; Delta" or "Recur ; Delta ;
Delta" take a date and add both deltas to it to give the starting and
ending time of the event.  The order and sign of the deltas is
unimportant (and both can be the same sign to give a range of times
which does not contain the base date).

=head1 KNOWN PROBLEMS

The following are not bugs in Date::Manip, but they may give some people
problems.

=over 4

=item B<Unable to determine Time Zone>

Perhaps the most common problem occurs when you get the error:

   Error: Date::Manip unable to determine Time Zone.

Date::Manip tries hard to determine the local time zone, but on some
machines, it cannot do this (especially non-Unix systems).  To fix this,
just set the TZ variable, either at the top of the Manip.pm file, in the
DateManip.cnf file, or in a call to Date_Init.  I suggest using the form
"EST5EDT" so you don't have to change it every 6 months when going to or
from daylight saving time.

Windows NT does not seem to set the time zone by default.  From the
Perl-Win32-Users mailing list:

   > How do I get the TimeZone on my NT?
   >
   >      $time_zone = $ENV{'TZ'};
   >
   You have to set the variable before, WinNT doesn't set it by
   default.  Open the properties of "My Computer" and set a SYSTEM
   variable TZ to your time zone.   Jenda@Krynicky.cz

This might help out some NT users.

A minor (false) assumption that some users might make is that since
Date::Manip passed all of its tests at install time, this should not occur
and are surprised when it does.

Some of the tests are time zone dependent.  Since the tests all include
input and expected output, I needed to know in advance what time zone they
would be run in.  So, the tests all explicitly set the time zone using the
TZ configuration variable passed into Date_Init.  Since this overrides any
other method of determining the time zone, Date::Manip uses this and doesn't
have to look elsewhere for the time zone.

When running outside the tests, Date::Manip has to rely on its other
methods for determining the time zone.

=item B<Missing date formats>

Please see the Date::Manip::Problems document for a discussion.

=item B<Complaining about getpwnam/getpwuid>

Another problem is when running on Micro$oft OS's.  I have added many
tests to catch them, but they still slip through occasionally.  If any ever
complain about getpwnam/getpwuid, simply add one of the lines:

  $ENV{OS} = Windows_NT
  $ENV{OS} = Windows_95

to your script before

  use Date::Manip

=item B<Date::Manip is slow>

The reasons for this are covered in the SHOULD I USE DATE::MANIP section
above.

Some things that will definitely help:

Version 5.21 does run noticeably faster than earlier versions due to
rethinking some of the initialization, so at the very least, make sure you
are running this version or later.

ISO-8601 dates are parsed first and fastest.  Use them whenever possible.

Avoid parsing dates that are referenced against the current time (in 2
days, today at noon, etc.).  These take a lot longer to parse.

   Example:  parsing 1065 dates with version 5.11 took 48.6 seconds, 36.2
   seconds with version 5.21, and parsing 1065 ISO-8601 dates with version
   5.21 took 29.1 seconds (these were run on a slow, overloaded computer with
   little memory... but the ratios should be reliable on a faster computer).

Business date calculations are extremely slow.  You should consider
alternatives if possible (i.e. doing the calculation in exact mode and
then multiplying by 5/7).  Who needs a business date more accurate
than "6 to 8 weeks" anyway, right :-)

Never call Date_Init more than once.  Unless you're doing something very
strange, there should never be a reason to anyway.

=item B<Sorting Problems>

If you use Date::Manip to sort a number of dates, you must call Date_Init
either explicitly, or by way of some other Date::Manip routine before it
is used in the sort.  For example, the following code fails:

   use Date::Manip;
   # Date_Init;
   sub sortDate {
       my($date1, $date2);
       $date1 = ParseDate($a);
       $date2 = ParseDate($b);
       return (Date_Cmp($date1,$date2));
   }
   @dates = ("Fri 16 Aug 96",
            "Mon 19 Aug 96",
            "Thu 15 Aug 96");
   @i=sort sortDate @dates;

but if you uncomment the Date_Init line, it works.  The reason for this is
that the first time you call Date_Init, it initializes a number of items
used by Date::Manip.  Some of these have to be sorted (regular expressions
sorted by length to ensure the longest match).  It turns out that Perl
has a bug in it which does not allow a sort within a sort.  At some point,
this should be fixed, but for now, the best thing to do is to call Date_Init
explicitly.  The bug exists in all versions up to 5.005 (I haven't
tested 5.6.0 yet).

NOTE: This is an EXTREMELY inefficient way to sort data (but read the 2nd
note below for an easy way to correct this).  Instead, you should parse the
dates with ParseDate, sort them using a normal string comparison, and then
convert them back to the format desired using UnixDate.

NOTE: It has been reported to me that you can still use ParseDate
to sort dates in this way, and be quite efficient through the use of
Memoize.  Just add the following lines to your code:

   use Date::Manip;
   use Memoize;
   memoize('ParseDate');
   ...
   @i=sort sortDate @dates;

Since sortDate would call ParseDate with the same data over and over,
this is a perfect application for the Memoize module.  So, sorting with
ParseDate is no longer slow for sorting.

=item B<RCS Control>

If you try to put Date::Manip under RCS control, you are going to have
problems.  Apparently, RCS replaces strings of the form "$Date...$" with
the current date.  This form occurs all over in Date::Manip.  To prevent the
RCS keyword expansion, checkout files using "co -ko".  Since very few people
will ever have a desire to do this (and I don't use RCS), I have not worried
about it.

=back

=head1 KNOWN BUGS

=over 4

=item B<Daylight Saving Times>

Date::Manip does not handle daylight saving time, though it does handle
time zones to a certain extent.  Converting from EST to PST works fine.
Going from EST to PDT is unreliable.

The following examples are run in the winter of the US East coast (i.e.
in the EST time zone).

	print UnixDate(ParseDate("6/1/97 noon"),"%u"),"\n";
        => Sun Jun  1 12:00:00 EST 1997

June 1 EST does not exist.  June 1st is during EDT.  It should print:

        => Sun Jun  1 00:00:00 EDT 1997

Even explicitly adding the time zone doesn't fix things (if anything, it
makes them worse):

	print UnixDate(ParseDate("6/1/97 noon EDT"),"%u"),"\n";
        => Sun Jun  1 11:00:00 EST 1997

Date::Manip converts everything to the current time zone (EST in this case).

Related problems occur when trying to do date calculations over a time zone
change.  These calculations may be off by an hour.

Also, if you are running a script which uses Date::Manip over a period of
time which starts in one time zone and ends in another (i.e. it switches
form Daylight Saving Time to Standard Time or vice versa), many things may
be wrong (especially elapsed time).

These problems will not be fixed in Date::Manip 5.xx. Date::Manip 6.xx has
full support for time zones and daylight saving time.

=back

=head1 BUGS AND QUESTIONS

Please refer to the Date::Manip::Problems documentation for
information on submitting bug reports or questions to the author.

=head1 SEE ALSO

Date::Manip        - main module documentation

=head1 LICENSE

This script is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

=head1 AUTHOR

Sullivan Beck (sbeck@cpan.org)

=cut