If you mean "convert a day ... to seconds since the epoch", you can try the Time::Local module, which I believe is part of the perl core distribution.

If you don't know what "seconds since the epoch" refers to, then I'm simply guessing wrong about what your question really means, and you should just ignore me and pay attention to the previous replies.

There are lots of CPAN modules in the "Date::" family, and some of them are pretty awe-inspiring (or overwhelming, depending on your nature). If you're doing arithmetic involving dates, you're bound to find a Date:: module that suits you. (But you can do quite a lot with just Time::Local, as well.)