gmtime - convert UNIX time into record or string using Greenwich time
Converts a time as returned by the time function to a 9-element array
with the time localized for the standard Greenwich timezone.
Typically used as follows:
All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6. If EXPR is omitted, does