1Time::Local(3pm) Perl Programmers Reference Guide Time::Local(3pm)
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6 Time::Local - efficiently compute time from local and GMT time
7
9 $time = timelocal($sec,$min,$hour,$mday,$mon,$year);
10 $time = timegm($sec,$min,$hour,$mday,$mon,$year);
11
13 This module provides functions that are the inverse of built-in perl
14 functions "localtime()" and "gmtime()". They accept a date as a six-
15 element array, and return the corresponding time(2) value in seconds
16 since the system epoch (Midnight, January 1, 1970 GMT on Unix, for
17 example). This value can be positive or negative, though POSIX only
18 requires support for positive values, so dates before the system's
19 epoch may not work on all operating systems.
20 It is worth drawing particular attention to the expected ranges for the
22 values provided. The value for the day of the month is the actual day
23 (ie 1..31), while the month is the number of months since January
24 (0..11). This is consistent with the values returned from "localtime()"
25 and "gmtime()".
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28 "timelocal()" and "timegm()"
29 This module exports two functions by default, "timelocal()" and
30 "timegm()".
31 The "timelocal()" and "timegm()" functions perform range checking on
33 the input $sec, $min, $hour, $mday, and $mon values by default.
34 "timelocal_nocheck()" and "timegm_nocheck()"
36 If you are working with data you know to be valid, you can speed your
37 code up by using the "nocheck" variants, "timelocal_nocheck()" and
38 "timegm_nocheck()". These variants must be explicitly imported.
39 use Time::Local 'timelocal_nocheck';
41 # The 365th day of 1999
43 print scalar localtime timelocal_nocheck 0,0,0,365,0,99;
44 If you supply data which is not valid (month 27, second 1,000) the
46 results will be unpredictable (so don't do that).
47 Year Value Interpretation
49 Strictly speaking, the year should be specified in a form consistent
50 with "localtime()", i.e. the offset from 1900. In order to make the
51 interpretation of the year easier for humans, however, who are more
52 accustomed to seeing years as two-digit or four-digit values, the
53 following conventions are followed:
54 · Years greater than 999 are interpreted as being the actual year,
56 rather than the offset from 1900. Thus, 1964 would indicate the
57 year Martin Luther King won the Nobel prize, not the year 3864.
58 · Years in the range 100..999 are interpreted as offset from 1900, so
60 that 112 indicates 2012. This rule also applies to years less than
61 zero (but see note below regarding date range).
62 · Years in the range 0..99 are interpreted as shorthand for years in
64 the rolling "current century," defined as 50 years on either side
65 of the current year. Thus, today, in 1999, 0 would refer to 2000,
66 and 45 to 2045, but 55 would refer to 1955. Twenty years from now,
67 55 would instead refer to 2055. This is messy, but matches the way
68 people currently think about two digit dates. Whenever possible,
69 use an absolute four digit year instead.
70 The scheme above allows interpretation of a wide range of dates,
72 particularly if 4-digit years are used.
73 Limits of time_t
75 The range of dates that can be actually be handled depends on the size
76 of "time_t" (usually a signed integer) on the given platform.
77 Currently, this is 32 bits for most systems, yielding an approximate
78 range from Dec 1901 to Jan 2038.
79 Both "timelocal()" and "timegm()" croak if given dates outside the
81 supported range.
82 Ambiguous Local Times (DST)
84 Because of DST changes, there are many time zones where the same local
85 time occurs for two different GMT times on the same day. For example,
86 in the "Europe/Paris" time zone, the local time of 2001-10-28 02:30:00
87 can represent either 2001-10-28 00:30:00 GMT, or 2001-10-28 01:30:00
88 GMT.
89 When given an ambiguous local time, the timelocal() function should
91 always return the epoch for the earlier of the two possible GMT times.
92 Non-Existent Local Times (DST)
94 When a DST change causes a locale clock to skip one hour forward, there
95 will be an hour's worth of local times that don't exist. Again, for the
96 "Europe/Paris" time zone, the local clock jumped from 2001-03-25
97 01:59:59 to 2001-03-25 03:00:00.
98 If the "timelocal()" function is given a non-existent local time, it
100 will simply return an epoch value for the time one hour later.
101 Negative Epoch Values
103 Negative epoch ("time_t") values are not officially supported by the
104 POSIX standards, so this module's tests do not test them. On some
105 systems, they are known not to work. These include MacOS (pre-OSX) and
106 Win32.
107 On systems which do support negative epoch values, this module should
109 be able to cope with dates before the start of the epoch, down the
110 minimum value of time_t for the system.
111
113 These routines are quite efficient and yet are always guaranteed to
114 agree with "localtime()" and "gmtime()". We manage this by caching the
115 start times of any months we've seen before. If we know the start time
116 of the month, we can always calculate any time within the month. The
117 start times are calculated using a mathematical formula. Unlike other
118 algorithms that do multiple calls to "gmtime()".
119 The "timelocal()" function is implemented using the same cache. We just
121 assume that we're translating a GMT time, and then fudge it when we're
122 done for the timezone and daylight savings arguments. Note that the
123 timezone is evaluated for each date because countries occasionally
124 change their official timezones. Assuming that "localtime()" corrects
125 for these changes, this routine will also be correct.
126
128 The whole scheme for interpreting two-digit years can be considered a
129 bug.
130
132 Support for this module is provided via the datetime@perl.org email
133 list. See http://lists.perl.org/ for more details.
134 Please submit bugs to the CPAN RT system at
136 http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Time-Local or via email
137 at bug-time-local@rt.cpan.org.
138
140 Copyright (c) 1997-2003 Graham Barr, 2003-2007 David Rolsky. All
141 rights reserved. This program is free software; you can redistribute
142 it and/or modify it under the same terms as Perl itself.
143 The full text of the license can be found in the LICENSE file included
145 with this module.
146
148 This module is based on a Perl 4 library, timelocal.pl, that was
149 included with Perl 4.036, and was most likely written by Tom
150 Christiansen.
151 The current version was written by Graham Barr.
153 It is now being maintained separately from the Perl core by Dave
155 Rolsky, <autarch@urth.org>.
156perl v5.10.1 2009-02-12 Time::Local(3pm)