Trying doing a Super Search for "dates" you will find much wisdom including references to- Time::Local (sample code), localtime(), the POSIX module (which is part of core Perl), various CPAN date modules including Date::Calc and Date::Manip.

Or leaving the Monastery for a moment a Google search for dealing with dates in perl would lead you to The Perl You Need to Know Part 9: Time and Date Manipulation

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Until you've lost your reputation, you never realize what a burden it was or what freedom really is. -Margaret Mitchell