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Time::Local(3pm) Perl Programmers Reference Guide Time::Local(3pm)
NAME
Time::Local - efficiently compute time from local and GMT time
SYNOPSIS
$time = timelocal($sec,$min,$hour,$mday,$mon,$year);
$time = timegm($sec,$min,$hour,$mday,$mon,$year);
DESCRIPTION
These routines 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 sup-
port 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 pro-
vided. The value for the day of the month is the actual day (ie 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().
The timelocal() and timegm() functions perform range checking on the input $sec,
$min, $hour, $mday, and $mon values by default. If you’d rather they didn’t, you
can explicitly import the timelocal_nocheck() and timegm_nocheck() functions.
use Time::Local ’timelocal_nocheck’;
{
# The 365th day of 1999
print scalar localtime timelocal_nocheck 0,0,0,365,0,99;
# The twenty thousandth day since 1970
print scalar localtime timelocal_nocheck 0,0,0,20000,0,70;
# And even the 10,000,000th second since 1999!
print scalar localtime timelocal_nocheck 10000000,0,0,1,0,99;
}
Your mileage may vary when trying these with minutes and hours, and it doesn’t work
at all for months.
Strictly speaking, the year should also 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, 1963 would indicate the year Martin Luther King
won the Nobel prize, not the year 3863.
· 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. When-
ever 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.
Please note, however, that the range of dates that can be actually be handled
depends on the size of an integer (time_t) on a 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.
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
Negative epoch (time_t) values are not officially supported by the POSIX standards,
so this module’s tests do not test them. On some systems, they are known not to
work. 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().
timelocal() is implemented using the same cache. We just assume that we’re trans-
lating 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 coun-
tries occasionally change their official timezones. Assuming that localtime() cor-
rects for these changes, this routine will also be correct.
BUGS
The whole scheme for interpreting two-digit years can be considered a bug.
SUPPORT
Support for this module is provided via the datetime AT perl.org email list. See
http://lists.perl.org/ for more details.
Please submit bugs using the RT system at rt.cpan.org, or as a last resort, to the
datetime AT perl.org list.
AUTHOR
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.
It is now being maintained separately from the Perl core by Dave Rolsky,
<autarch AT urth.org>.
perl v5.8.6 2001-09-21 Time::Local(3pm)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 04:21 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)