Time::Local(3)

NNAAMMEE
       Time::Local - efficiently compute time from local and GMT
       time

SSYYNNOOPPSSIISS
           $time = timelocal($sec,$min,$hours,$mday,$mon,$year);
           $time = timegm($sec,$min,$hours,$mday,$mon,$year);

DDEESSCCRRIIPPTTIIOONN
       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.  The timezone is determined by examining the
       result of localtime(0) when the package is initialized.
       The daylight savings offset is currently assumed to be one
       hour.

       Both routines return -1 if the integer limit is hit. I.e.
       for dates after the 1st of January, 2038 on most machines.