Time::Local(3perl)

📦
version 5.44.0 · 14 versions
, in arch rolling

✨ NAME ✨

Time::Local — bestie, this is the module that computes time from local time and GMT time, and it does it efficiently. 💖 (This page lives in the Perl Programmers Reference Guide, section 3perl, just so you know where you are. 🫶)

🌸 VERSION 🌸

Like, version 1.35. That's the one. Periodt.

🎀 SYNOPSIS 🎀

use Time::Local qw( timelocal_posix timegm_posix );

my $time = timelocal_posix( $sec, $min, $hour, $mday, $mon, $year );
my $time = timegm_posix( $sec, $min, $hour, $mday, $mon, $year );

Look at her. So clean. So iconic. ✨

🦋 DESCRIPTION 🦋

Okay so basically — this module gives you functions that are the exact inverse of perl's built-in localtime() and gmtime(). You hand them a date as a six-element array, and they hand you back the matching time(2) value in seconds since the system epoch (like, Midnight, January 1, 1970 GMT on Unix, for the vibe). 🌈

That value can be positive or negative, though ngl — POSIX only requires support for positive values, so dates before the system's epoch might just not work on every operating system. Not all platforms are besties about it. 😭

Real talk though: the expected ranges matter so much. 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). That's consistent with what localtime() and gmtime() return, so it's giving "matches the builtins" energy. 💅

👑 FUNCTIONS 👑

💖 timelocal_posix() and timegm_posix()

Since version 1.30, bestie. ✨

These are the exact inverse of Perl's built-in localtime and gmtime functions. Which means — and this is the iconic part — calling timelocal_posix( localtime($value) ) will always give you the same $value you started with. Same deal for timegm_posix( gmtime($value) ). Round trip? Slayed. 🔥

The one exception? When the value returned from localtime() represents an ambiguous local time because of a DST change. More on that below — go read the docs, iykyk. 🕰️

These functions expect the year value to be the number of years since 1900, which is exactly what the localtime() and gmtime() built-ins return.

They do range checking by default on the input $sec, $min, $hour, $mday, and $mon values, and they will croak (using Carp::croak()) if you hand them anything outside the allowed ranges. 💀

Now — it would be so nice to make this the default behavior, but that would almost certainly break a LOT of code, so you have to explicitly import these functions and use them instead of the default timelocal() and timegm().

You are strongly encouraged to use these functions in any new code which uses this module. It will almost certainly make your code's behavior less surprising. Trust. 🫶

🧚 timelocal_modern() and timegm_modern()

Since version 1.27. 💜

Back when Time::Local was first written, it was a common practice to represent years as a two-digit value like 99 for 1999 or 1 for 2001. This caused all sorts of problems (google "Y2K problem" if you're very young, no cap) and developers eventually realized this was a terrible idea. 🙈

The default exports of timelocal() and timegm() do a complicated calculation when given a year value less than 1000. That leads to surprising results in many cases. See "Year Value Interpretation" for the details — it's a whole thing.

The time*_modern() functions do not do this year munging. They simply take the year value as provided. Simple. Clean. Main character energy. 👑

They perform range checking by default on the input $sec, $min, $hour, $mday, and $mon values and will croak (using Carp::croak()) if given a value outside the allowed ranges.

🌸 timelocal() and timegm()

This module exports two functions by default — timelocal() and timegm().

They perform range checking by default on the input $sec, $min, $hour, $mday, and $mon values and will croak (using Carp::croak()) if given a value outside the allowed ranges.

⚠️ Warning: The year value interpretation that these functions and their nocheck variants use will almost certainly lead to bugs in your code — if not now, then in the future. You are strongly discouraged from using these in new code, and you should convert old code to using either the *_posix or *_modern functions if possible. This is not a drill, bestie. 😭

⭐ timelocal_nocheck() and timegm_nocheck()

If you're working with data you know to be valid, you can use the nocheck variants, timelocal_nocheck() and timegm_nocheck(). These variants must be explicitly imported.

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

Note that the benchmarks show this is just a 3% speed increase over the checked versions, so unless calling Time::Local is the hottest spot in your application, using these nocheck variants is unlikely to have much impact. Like, it's giving "marginal gains" not "glow up". 💅

🎀 Year Value Interpretation 🎀

This does not apply to the *_posix or *_modern functions. Use those exports if you want to ensure consistent behavior as your code ages. (Aging code? We don't know her. ✨)

Strictly speaking, the year should be specified in a form consistent with localtime(), i.e. the offset from 1900. But — to make the interpretation of the year easier for humans, 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 the 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. But it also means that the behavior of your code changes as time passes, because the rolling "current century" changes each year. Chaos, honestly. 😵‍💫

🦋 Limits of time_t 🦋

On perl versions older than 5.12.0, the range of dates that can 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 time implementation of the operating system it's running on. Instead, it has its own implementation of those routines with a safe range of at least +/- 2**52 (about 142 million years). Yeah. Eat that. 🔥

💜 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. Two timelines, one clock. It's giving multiverse. 🌌

When given an ambiguous local time, the timelocal() function will 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. That hour just... vanished. Gone. Not invited. 💨

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. Slay. ✨

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(). (Not us being shady, but. 💅)

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. Yeah. We said it. 🙈

Bugs may be submitted at https://github.com/houseabsolute/Time-Local/issues.

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

🎀 SOURCE 🎀

The source code repository for Time-Local can be found at https://github.com/houseabsolute/Time-Local.

⭐ AUTHOR ⭐

Dave Rolsky <autarch@urth.org>

🫶 CONTRIBUTORS 🫶

  • Florian Ragwitz <rafl@debian.org>
  • Gregory Oschwald <oschwald@gmail.com>
  • J. Nick Koston <nick@cpanel.net>
  • Tom Wyant <wyant@cpan.org>
  • Unknown <unknown@example.com>

This software is copyright (c) 1997 - 2023 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.

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

okay that's the whole page, bestie, you made it to the end 💅✨ This tea was brewed by perl v5.44.0 on 2026-10-17 — and yeah, that's the whole Time::Local(3perl) shebang. Go write some dates that don't lie to you. 🦋💖

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