# NAME

Time::Local - Efficiently compute time from local and GMT time

# VERSION

version 1.25

# SYNOPSIS

    use Time::Local;

    my $time = timelocal( $sec, $min, $hour, $mday, $mon, $year );
    my $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 (i.e. 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.

# AUTHORS EMERITUS

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.

# BUGS

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

Bugs may be submitted through [https://github.com/houseabsolute/Time-Local/issues](https://github.com/houseabsolute/Time-Local/issues).

There is a mailing list available for users of this distribution,
[mailto:datetime@perl.org](mailto:datetime@perl.org).

I am also usually active on IRC as 'autarch' on `irc://irc.perl.org`.

# AUTHOR

Dave Rolsky <autarch@urth.org>

# CONTRIBUTORS

- Florian Ragwitz <rafl@debian.org>
- J. Nick Koston <nick@cpanel.net>
- Unknown <unknown@example.com>

# COPYRIGHT AND LICENSE

This software is copyright (c) 1997 - 2016 by Graham Barr & Dave Rolsky.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.