


Time::Local(3p)  Perl Programmers Reference Guide Time::Local(3p)


NAME
       Time::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:

       o   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



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Time::Local(3p)  Perl Programmers Reference Guide Time::Local(3p)


           the Nobel prize, not the year 2863.

       o   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).

       o   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.





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Time::Local(3p)  Perl Programmers Reference Guide Time::Local(3p)


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.

















































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