


Time::Local(3p)  Perl Programmers Reference Guide Time::Local(3p)


NAME
       Time::Local - efficiently compute time from local and GMT
       time

SYNOPSIS
           $time = timelocal($sec,$min,$hour,$mday,$mon,$year);
           $time = timegm($sec,$min,$hour,$mday,$mon,$year);

DESCRIPTION
       This module provides functions that are the inverse of
       built-in perl functions "localtime()" and "gmtime()". They
       accept a date as a six-element array, and return the
       corresponding time(2) value in seconds since the system
       epoch (Midnight, January 1, 1970 GMT on Unix, for
       example). This value can be positive or negative, though
       POSIX only requires support for positive values, so dates
       before the system's epoch may not work on all operating
       systems.

       It is worth drawing particular attention to the expected
       ranges for the values provided. The value for the day of
       the month is the actual day (ie 1..31), while the month is
       the number of months since January (0..11). This is
       consistent with the values returned from "localtime()" and
       "gmtime()".

FUNCTIONS
   "timelocal()" and "timegm()"
       This module exports two functions by default,
       "timelocal()" and "timegm()".

       The "timelocal()" and "timegm()" functions perform range
       checking on the input $sec, $min, $hour, $mday, and $mon
       values by default.

   "timelocal_nocheck()" and "timegm_nocheck()"
       If you are working with data you know to be valid, you can
       speed your code up by using the "nocheck" variants,
       "timelocal_nocheck()" and "timegm_nocheck()". These
       variants must be explicitly imported.

           use Time::Local 'timelocal_nocheck';

           # The 365th day of 1999
           print scalar localtime timelocal_nocheck 0,0,0,365,0,99;

       If you supply data which is not valid (month 27, second
       1,000) the results will be unpredictable (so don't do
       that).

   Year Value Interpretation
       Strictly speaking, the year should be specified in a form
       consistent with "localtime()", i.e. the offset from 1900.
       In order to make the interpretation of the year easier for



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       humans, however, who are more accustomed to seeing years
       as two-digit or four-digit values, the following
       conventions are followed:

       o   Years greater than 999 are interpreted as being the
           actual year, rather than the offset from 1900. Thus,
           1964 would indicate the year Martin Luther King won
           the Nobel prize, not the year 3864.

       o   Years in the range 100..999 are interpreted as offset
           from 1900, so that 112 indicates 2012. This rule also
           applies to years less than zero (but see note below
           regarding date range).

       o   Years in the range 0..99 are interpreted as shorthand
           for years in the rolling "current century," defined as
           50 years on either side of the current year. Thus,
           today, in 1999, 0 would refer to 2000, and 45 to 2045,
           but 55 would refer to 1955. Twenty years from now, 55
           would instead refer to 2055. This is messy, but
           matches the way people currently think about two digit
           dates. Whenever possible, use an absolute four digit
           year instead.

       The scheme above allows interpretation of a wide range of
       dates, particularly if 4-digit years are used.

   Limits of time_t
       The range of dates that can be actually be handled depends
       on the size of "time_t" (usually a signed integer) on the
       given platform. Currently, this is 32 bits for most
       systems, yielding an approximate range from Dec 1901 to
       Jan 2038.

       Both "timelocal()" and "timegm()" croak if given dates
       outside the supported range.

   Ambiguous Local Times (DST)
       Because of DST changes, there are many time zones where
       the same local time occurs for two different GMT times on
       the same day. For example, in the "Europe/Paris" time
       zone, the local time of 2001-10-28 02:30:00 can represent
       either 2001-10-28 00:30:00 GMT, or 2001-10-28 01:30:00
       GMT.

       When given an ambiguous local time, the timelocal()
       function should always return the epoch for the earlier of
       the two possible GMT times.

   Non-Existent Local Times (DST)
       When a DST change causes a locale clock to skip one hour
       forward, there will be an hour's worth of local times that
       don't exist. Again, for the "Europe/Paris" time zone, the
       local clock jumped from 2001-03-25 01:59:59 to 2001-03-25



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Time::Local(3p)  Perl Programmers Reference Guide Time::Local(3p)


       03:00:00.

       If the "timelocal()" function is given a non-existent
       local time, it will simply return an epoch value for the
       time one hour later.

   Negative Epoch Values
       Negative epoch ("time_t") values are not officially
       supported by the POSIX standards, so this module's tests
       do not test them. On some systems, they are known not to
       work. These include MacOS (pre-OSX) and Win32.

       On systems which do support negative epoch values, this
       module should be able to cope with dates before the start
       of the epoch, down the minimum value of time_t for the
       system.

IMPLEMENTATION
       These routines are quite efficient and yet are always
       guaranteed to agree with "localtime()" and "gmtime()". We
       manage this by caching the start times of any months we've
       seen before. If we know the start time of the month, we
       can always calculate any time within the month.  The start
       times are calculated using a mathematical formula. Unlike
       other algorithms that do multiple calls to "gmtime()".

       The "timelocal()" function is implemented using the same
       cache. We just assume that we're translating a GMT time,
       and then fudge it when we're done for the timezone and
       daylight savings arguments. Note that the timezone is
       evaluated for each date because countries occasionally
       change their official timezones. Assuming that
       "localtime()" corrects for these changes, this routine
       will also be correct.

BUGS
       The whole scheme for interpreting two-digit years can be
       considered a bug.

SUPPORT
       Support for this module is provided via the
       datetime@perl.org email list. See http://lists.perl.org/
       for more details.

       Please submit bugs to the CPAN RT system at
       http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Time-Local
       or via email at bug-time-local@rt.cpan.org.

COPYRIGHT
       Copyright (c) 1997-2003 Graham Barr, 2003-2007 David
       Rolsky.  All rights reserved.  This program is free
       software; you can redistribute it and/or modify it under
       the same terms as Perl itself.




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       The full text of the license can be found in the LICENSE
       file included with this module.

AUTHOR
       This module is based on a Perl 4 library, timelocal.pl,
       that was included with Perl 4.036, and was most likely
       written by Tom Christiansen.

       The current version was written by Graham Barr.

       It is now being maintained separately from the Perl core
       by Dave Rolsky, <autarch@urth.org>.













































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