1Time::Local(3) User Contributed Perl Documentation Time::Local(3)
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6 Time::Local - Efficiently compute time from local and GMT time
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9 version 1.30
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12 use Time::Local qw( timelocal_posix timegm_posix );
13 my $time = timelocal_posix( $sec, $min, $hour, $mday, $mon, $year );
15 my $time = timegm_posix( $sec, $min, $hour, $mday, $mon, $year );
16
18 This module provides functions that are the inverse of built-in perl
19 functions "localtime()" and "gmtime()". They accept a date as a six-
20 element array, and return the corresponding time(2) value in seconds
21 since the system epoch (Midnight, January 1, 1970 GMT on Unix, for
22 example). This value can be positive or negative, though POSIX only
23 requires support for positive values, so dates before the system's
24 epoch may not work on all operating systems.
25 It is worth drawing particular attention to the expected ranges for the
27 values provided. The value for the day of the month is the actual day
28 (i.e. 1..31), while the month is the number of months since January
29 (0..11). This is consistent with the values returned from "localtime()"
30 and "gmtime()".
31
33 "timelocal_posix()" and "timegm_posix()"
34 These functions are the exact inverse of Perl's built-in "localtime"
35 and "gmtime" functions. That means that calling "timelocal_posix(
36 localtime($value) )" will always give you the same $value you started
37 with. The same applies to "timegm_posix( gmtime($value) )".
38 The one exception is when the value returned from "localtime()"
40 represents an ambiguous local time because of a DST change. See the
41 documentation below for more details.
42 These functions expect the year value to be the number of years since
44 1900, which is what the "localtime()" and "gmtime()" built-ins returns.
45 They perform range checking by default on the input $sec, $min, $hour,
47 $mday, and $mon values and will croak (using "Carp::croak()") if given
48 a value outside the allowed ranges.
49 While it would be nice to make this the default behavior, that would
51 almost certainly break a lot of code, so you must explicitly import
52 these functions and use them instead of the default "timelocal()" and
53 "timegm()".
54 You are strongly encouraged to use these functions in any new code
56 which uses this module. It will almost certainly make your code's
57 behavior less surprising.
58 "timelocal_modern()" and "timegm_modern()"
60 When "Time::Local" was first written, it was a common practice to
61 represent years as a two-digit value like 99 for 1999 or 1 for 2001.
62 This caused all sorts of problems (google "Y2K problem" if you're very
63 young) and developers eventually realized that this was a terrible
64 idea.
65 The default exports of "timelocal()" and "timegm()" do a complicated
67 calculation when given a year value less than 1000. This leads to
68 surprising results in many cases. See "Year Value Interpretation" for
69 details.
70 The "time*_modern()" functions do not do this year munging and simply
72 take the year value as provided.
73 They perform range checking by default on the input $sec, $min, $hour,
75 $mday, and $mon values and will croak (using "Carp::croak()") if given
76 a value outside the allowed ranges.
77 "timelocal()" and "timegm()"
79 This module exports two functions by default, "timelocal()" and
80 "timegm()".
81 They perform range checking by default on the input $sec, $min, $hour,
83 $mday, and $mon values and will croak (using "Carp::croak()") if given
84 a value outside the allowed ranges.
85 Warning: The year value interpretation that these functions and their
87 nocheck variants use will almost certainly lead to bugs in your code,
88 if not now, then in the future. You are strongly discouraged from using
89 these in new code, and you should convert old code to using either the
90 *_posix or *_modern functions if possible.
91 "timelocal_nocheck()" and "timegm_nocheck()"
93 If you are working with data you know to be valid, you can use the
94 "nocheck" variants, "timelocal_nocheck()" and "timegm_nocheck()". These
95 variants must be explicitly imported.
96 If you supply data which is not valid (month 27, second 1,000) the
98 results will be unpredictable (so don't do that).
99 Note that my benchmarks show that this is just a 3% speed increase over
101 the checked versions, so unless calling "Time::Local" is the hottest
102 spot in your application, using these nocheck variants is unlikely to
103 have much impact on your application.
104 Year Value Interpretation
106 This does not apply to the *_posix or *_modern functions. Use those
107 exports if you want to ensure consistent behavior as your code ages.
108 Strictly speaking, the year should be specified in a form consistent
110 with "localtime()", i.e. the offset from 1900. In order to make the
111 interpretation of the year easier for humans, however, who are more
112 accustomed to seeing years as two-digit or four-digit values, the
113 following conventions are followed:
114 • Years greater than 999 are interpreted as being the actual year,
116 rather than the offset from 1900. Thus, 1964 would indicate the
117 year Martin Luther King won the Nobel prize, not the year 3864.
118 • Years in the range 100..999 are interpreted as offset from 1900, so
120 that 112 indicates 2012. This rule also applies to years less than
121 zero (but see note below regarding date range).
122 • Years in the range 0..99 are interpreted as shorthand for years in
124 the rolling "current century," defined as 50 years on either side
125 of the current year. Thus, today, in 1999, 0 would refer to 2000,
126 and 45 to 2045, but 55 would refer to 1955. Twenty years from now,
127 55 would instead refer to 2055. This is messy, but matches the way
128 people currently think about two digit dates. Whenever possible,
129 use an absolute four digit year instead.
130 The scheme above allows interpretation of a wide range of dates,
132 particularly if 4-digit years are used. But it also means that the
133 behavior of your code changes as time passes, because the rolling
134 "current century" changes each year.
135 Limits of time_t
137 On perl versions older than 5.12.0, the range of dates that can be
138 actually be handled depends on the size of "time_t" (usually a signed
139 integer) on the given platform. Currently, this is 32 bits for most
140 systems, yielding an approximate range from Dec 1901 to Jan 2038.
141 Both "timelocal()" and "timegm()" croak if given dates outside the
143 supported range.
144 As of version 5.12.0, perl has stopped using the time implementation of
146 the operating system it's running on. Instead, it has its own
147 implementation of those routines with a safe range of at least +/-
148 2**52 (about 142 million years)
149 Ambiguous Local Times (DST)
151 Because of DST changes, there are many time zones where the same local
152 time occurs for two different GMT times on the same day. For example,
153 in the "Europe/Paris" time zone, the local time of 2001-10-28 02:30:00
154 can represent either 2001-10-28 00:30:00 GMT, or 2001-10-28 01:30:00
155 GMT.
156 When given an ambiguous local time, the timelocal() function will
158 always return the epoch for the earlier of the two possible GMT times.
159 Non-Existent Local Times (DST)
161 When a DST change causes a locale clock to skip one hour forward, there
162 will be an hour's worth of local times that don't exist. Again, for the
163 "Europe/Paris" time zone, the local clock jumped from 2001-03-25
164 01:59:59 to 2001-03-25 03:00:00.
165 If the "timelocal()" function is given a non-existent local time, it
167 will simply return an epoch value for the time one hour later.
168 Negative Epoch Values
170 On perl version 5.12.0 and newer, negative epoch values are fully
171 supported.
172 On older versions of perl, negative epoch ("time_t") values, which are
174 not officially supported by the POSIX standards, are known not to work
175 on some systems. These include MacOS (pre-OSX) and Win32.
176 On systems which do support negative epoch values, this module should
178 be able to cope with dates before the start of the epoch, down the
179 minimum value of time_t for the system.
180
182 These routines are quite efficient and yet are always guaranteed to
183 agree with "localtime()" and "gmtime()". We manage this by caching the
184 start times of any months we've seen before. If we know the start time
185 of the month, we can always calculate any time within the month. The
186 start times are calculated using a mathematical formula. Unlike other
187 algorithms that do multiple calls to "gmtime()".
188 The "timelocal()" function is implemented using the same cache. We just
190 assume that we're translating a GMT time, and then fudge it when we're
191 done for the timezone and daylight savings arguments. Note that the
192 timezone is evaluated for each date because countries occasionally
193 change their official timezones. Assuming that "localtime()" corrects
194 for these changes, this routine will also be correct.
195
197 This module is based on a Perl 4 library, timelocal.pl, that was
198 included with Perl 4.036, and was most likely written by Tom
199 Christiansen.
200 The current version was written by Graham Barr.
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204 The whole scheme for interpreting two-digit years can be considered a
205 bug.
206 Bugs may be submitted at
208 <https://github.com/houseabsolute/Time-Local/issues>.
209 There is a mailing list available for users of this distribution,
211 <mailto:datetime@perl.org>.
212 I am also usually active on IRC as 'autarch' on "irc://irc.perl.org".
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216 The source code repository for Time-Local can be found at
217 <https://github.com/houseabsolute/Time-Local>.
218
220 Dave Rolsky <autarch@urth.org>
221
223 • Florian Ragwitz <rafl@debian.org>
224 • J. Nick Koston <nick@cpanel.net>
226 • Unknown <unknown@example.com>
228
230 This software is copyright (c) 1997 - 2020 by Graham Barr & Dave
231 Rolsky.
232 This is free software; you can redistribute it and/or modify it under
234 the same terms as the Perl 5 programming language system itself.
235 The full text of the license can be found in the LICENSE file included
237 with this distribution.
238perl v5.34.0 2021-07-23 Time::Local(3)