Time::Local(3) Perl Programmers Reference Guide Time::Local(3)NAMETime::Local - efficiently compute time from local and GMT
time
SYNOPSIS
$time = timelocal($sec,$min,$hours,$mday,$mon,$year);
$time = timegm($sec,$min,$hours,$mday,$mon,$year);
DESCRIPTION
These routines are the inverse of built-in perl fuctions
localtime() and gmtime(). They accept a date as a six-
element array, and return the corresponding time(2) value
in seconds since the Epoch (Midnight, January 1, 1970).
This value can be positive or negative.
It is worth drawing particular attention to the expected
ranges for the values provided. While the day of the
month is expected to be in the range 1..31, the month
should be in the range 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, $hours, $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 fol
lowing 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 2863.
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. 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.
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.
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 themselves are guessed by successive approximation
starting at the current time, since most dates seen in
practice are close to the current date. Unlike algorithms
that do a binary search (calling gmtime once for each bit
of the time value, resulting in 32 calls), this algorithm
calls it at most 6 times, and usually only once or twice.
If you hit the month cache, of course, it doesn't call it
at all.
timelocal() 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. The daylight
savings offset is currently assumed to be one hour.
BUGS
The whole scheme for interpreting two-digit years can be
considered a bug.
Note that the cache currently handles only years from 1900
through 2155.
The proclivity to croak() is probably a bug.
2001-02-22 perl v5.6.1 Time::Local(3)
