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.

-rw-r--r--    1 root    root            22138 aoû 26  1997 /usr/lib/perl5/vendor_perl/5.8.1/Time/Period.pm
drwxr-xr-x    2 root    root                0 aoû 13  2003 /usr/share/doc/perl-Period-1.20
-rw-r--r--    1 root    root             7543 aoû 26  1997 /usr/share/doc/perl-Period-1.20/Period.html
-rw-r--r--    1 root    root             2373 aoû 26  1997 /usr/share/doc/perl-Period-1.20/README
-rw-r--r--    1 root    root             4473 aoû 13  2003 /usr/share/man/man3/Time::Period.3pm.bz2