I guess this isn't what you want to hear, but the best way to do this without installing a module is to use a module.
There exists a nice date module Date::Manip which will do almost anything with dates, and which (thanks to it being entirely Perl) you can simple copy the module's .pm files into the place you're installing your own code and it.
In general date usage there are lighter and quicker modules that'll get the job done just as well, but for use when you can't install anything this module is the one to go for.
Update: Slight clarification as to how to use it without needing to install it.
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