I have come to love the DateTime modules for this kind of work. They're lean, offer a clean interface, and allow you to do just about anything you'd ever want to, without much hassle (so long as you don't mind occasionally reading the docs).

If you want a sledgehammer solution, try Date::Manip — its original intent was to let people enter complex time specifications like "second Wednesday this month" on the commandline and have them work. It's rather heavy and the author himself discourages its use for more narrowly defined scenarios, though.