Check out Date::Manip, which allows you to comparisons and calculations with fixed dates/times and differences. There's also modules like Date::Calc.

Basically, do a CPAN search for anything Date- and Time-related and you'll get a whole slew of different date/time modules that mostly do the same things, but each a little differently. I'm trying to avoid getting dozens of posts in reply to this question with everyone's favorite date module. :)