Period.pm is a Perl module that contains code to deal with time periods.
Currently, there is only a function in this module.  That function is
called inPeriod().

inPeriod() determines if a given time is within a given time period.
It will return 1 if it is, 0 if not, and -1 if either the time or the
period passed to it were malformed.  The time is specified in non-leap
year seconds past January 1, 1970, as per the time() function.  The period
is a string which is of the form described in Period's man page.