All the *good* answers to this question are going to involve some module or another. Otherwise, you're going to have to basically duplicate the (relevant) functionality of the module in your code.

The module doesn't have to be Date::Calc necessarily. There are numerous other options. (I would use DateTime myself.) You *may* even be able to find one that's pure Perl and doesn't have a lot of dependencies, so that you can just place a copy of the .pm in the same directory as your code and not have to install it per se. (DateTime is not the answer in this respect; it has a trainload of dependencies; the reason I use it is because it does everything. The DateTime::Format modules are particularly useful. But that stuff is beyond your needs just at the moment.) Poke around on search.cpan.org with search strings like "date" and "time" and "timezone" and see if you can find a _simple_ module that's pure perl and just does what you want.