There are plenty of date manipulation modules on CPAN. You might start with Class::Date, as it allows you to add and subtract days, years, etc.

Update: After reading Beatnik's post, I recall that I have used Date::Calc before, and as he points out, it will do what you're needing to do. It's also far more straightforward and easy to use (IMO) than Class::Date.

If things get any worse, I'll have to ask you to stop helping me.