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 humans, however, who are more accustomed
to seeing years as two-digit or four-digit values, the following conventions
are followed:
- •
- 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.
- •
- 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).
- •
- 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¶
On perl versions older than 5.12.0, 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.
As of version 5.12.0, perl has stopped using the underlying time library of the
operating system it's running on and has its own implementation of those
routines with a safe range of at least +/ 2**52 (about 142 million years).
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 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¶
On perl version 5.12.0 and newer, negative epoch values are fully supported.
On older versions of perl, negative epoch ("time_t") values, which are
not officially supported by the POSIX standards, are known not to work on some
systems. 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.
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>.