1Time::Local(3)        User Contributed Perl Documentation       Time::Local(3)
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NAME

6       Time::Local - Efficiently compute time from local and GMT time
7

VERSION

9       version 1.28
10

SYNOPSIS

12           use Time::Local;
13
14           my $time = timelocal( $sec, $min, $hour, $mday, $mon, $year );
15           my $time = timegm( $sec, $min, $hour, $mday, $mon, $year );
16

DESCRIPTION

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
26       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

FUNCTIONS

33   "timelocal_modern()" and "timegm_modern()"
34       When "Time::Local" was first written, it was a common practice to
35       represent years as a two-digit value like 99 for 1999 or 1 for 2001.
36       This caused all sorts of problems (google "Y2K problem" if you're very
37       young) and developers eventually realized that this was a terrible
38       idea.
39
40       The default exports of "timelocal()" and "timegm()" do a complicated
41       calculation when given a year value less than 1000. This leads to
42       surprising results in many cases. See "Year Value Interpretation" for
43       details.
44
45       The "time*_modern()" subs do not do this year munging and simply take
46       the year value as provided.
47
48       While it would be nice to make this the default behavior, that would
49       almost certainly break a lot of code, so you must explicitly import
50       these subs and use them instead of the default "timelocal()" and
51       "timegm()".
52
53       You are strongly encouraged to use these subs in any new code which
54       uses this module. It will almost certainly make your code's behavior
55       less surprising.
56
57   "timelocal()" and "timegm()"
58       This module exports two functions by default, "timelocal()" and
59       "timegm()".
60
61       The "timelocal()" and "timegm()" functions perform range checking on
62       the input $sec, $min, $hour, $mday, and $mon values by default.
63
64   "timelocal_nocheck()" and "timegm_nocheck()"
65       If you are working with data you know to be valid, you can speed your
66       code up by using the "nocheck" variants, "timelocal_nocheck()" and
67       "timegm_nocheck()". These variants must be explicitly imported.
68
69           use Time::Local 'timelocal_nocheck';
70
71           # The 365th day of 1999
72           print scalar localtime timelocal_nocheck( 0, 0, 0, 365, 0, 99 );
73
74       If you supply data which is not valid (month 27, second 1,000) the
75       results will be unpredictable (so don't do that).
76
77   Year Value Interpretation
78       This does not apply to "timelocal_modern" or "timegm_modern". Use those
79       exports if you want to ensure consistent behavior as your code ages.
80
81       Strictly speaking, the year should be specified in a form consistent
82       with "localtime()", i.e. the offset from 1900. In order to make the
83       interpretation of the year easier for humans, however, who are more
84       accustomed to seeing years as two-digit or four-digit values, the
85       following conventions are followed:
86
87       ·   Years greater than 999 are interpreted as being the actual year,
88           rather than the offset from 1900. Thus, 1964 would indicate the
89           year Martin Luther King won the Nobel prize, not the year 3864.
90
91       ·   Years in the range 100..999 are interpreted as offset from 1900, so
92           that 112 indicates 2012. This rule also applies to years less than
93           zero (but see note below regarding date range).
94
95       ·   Years in the range 0..99 are interpreted as shorthand for years in
96           the rolling "current century," defined as 50 years on either side
97           of the current year. Thus, today, in 1999, 0 would refer to 2000,
98           and 45 to 2045, but 55 would refer to 1955. Twenty years from now,
99           55 would instead refer to 2055. This is messy, but matches the way
100           people currently think about two digit dates. Whenever possible,
101           use an absolute four digit year instead.
102
103       The scheme above allows interpretation of a wide range of dates,
104       particularly if 4-digit years are used.
105
106   Limits of time_t
107       On perl versions older than 5.12.0, the range of dates that can be
108       actually be handled depends on the size of "time_t" (usually a signed
109       integer) on the given platform. Currently, this is 32 bits for most
110       systems, yielding an approximate range from Dec 1901 to Jan 2038.
111
112       Both "timelocal()" and "timegm()" croak if given dates outside the
113       supported range.
114
115       As of version 5.12.0, perl has stopped using the time implementation of
116       the operating system it's running on. Instead, it has its own
117       implementation of those routines with a safe range of at least +/-
118       2**52 (about 142 million years)
119
120   Ambiguous Local Times (DST)
121       Because of DST changes, there are many time zones where the same local
122       time occurs for two different GMT times on the same day. For example,
123       in the "Europe/Paris" time zone, the local time of 2001-10-28 02:30:00
124       can represent either 2001-10-28 00:30:00 GMT, or 2001-10-28 01:30:00
125       GMT.
126
127       When given an ambiguous local time, the timelocal() function should
128       always return the epoch for the earlier of the two possible GMT times.
129
130   Non-Existent Local Times (DST)
131       When a DST change causes a locale clock to skip one hour forward, there
132       will be an hour's worth of local times that don't exist. Again, for the
133       "Europe/Paris" time zone, the local clock jumped from 2001-03-25
134       01:59:59 to 2001-03-25 03:00:00.
135
136       If the "timelocal()" function is given a non-existent local time, it
137       will simply return an epoch value for the time one hour later.
138
139   Negative Epoch Values
140       On perl version 5.12.0 and newer, negative epoch values are fully
141       supported.
142
143       On older versions of perl, negative epoch ("time_t") values, which are
144       not officially supported by the POSIX standards, are known not to work
145       on some systems. These include MacOS (pre-OSX) and Win32.
146
147       On systems which do support negative epoch values, this module should
148       be able to cope with dates before the start of the epoch, down the
149       minimum value of time_t for the system.
150

IMPLEMENTATION

152       These routines are quite efficient and yet are always guaranteed to
153       agree with "localtime()" and "gmtime()". We manage this by caching the
154       start times of any months we've seen before. If we know the start time
155       of the month, we can always calculate any time within the month.  The
156       start times are calculated using a mathematical formula. Unlike other
157       algorithms that do multiple calls to "gmtime()".
158
159       The "timelocal()" function is implemented using the same cache. We just
160       assume that we're translating a GMT time, and then fudge it when we're
161       done for the timezone and daylight savings arguments. Note that the
162       timezone is evaluated for each date because countries occasionally
163       change their official timezones. Assuming that "localtime()" corrects
164       for these changes, this routine will also be correct.
165

AUTHORS EMERITUS

167       This module is based on a Perl 4 library, timelocal.pl, that was
168       included with Perl 4.036, and was most likely written by Tom
169       Christiansen.
170
171       The current version was written by Graham Barr.
172

BUGS

174       The whole scheme for interpreting two-digit years can be considered a
175       bug.
176
177       Bugs may be submitted at
178       <https://github.com/houseabsolute/Time-Local/issues>.
179
180       There is a mailing list available for users of this distribution,
181       <mailto:datetime@perl.org>.
182
183       I am also usually active on IRC as 'autarch' on "irc://irc.perl.org".
184

SOURCE

186       The source code repository for Time-Local can be found at
187       <https://github.com/houseabsolute/Time-Local>.
188

AUTHOR

190       Dave Rolsky <autarch@urth.org>
191

CONTRIBUTORS

193       ·   Florian Ragwitz <rafl@debian.org>
194
195       ·   J. Nick Koston <nick@cpanel.net>
196
197       ·   Unknown <unknown@example.com>
198
200       This software is copyright (c) 1997 - 2018 by Graham Barr & Dave
201       Rolsky.
202
203       This is free software; you can redistribute it and/or modify it under
204       the same terms as the Perl 5 programming language system itself.
205
206       The full text of the license can be found in the LICENSE file included
207       with this distribution.
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211perl v5.28.1                      2018-06-13                    Time::Local(3)
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