I don't think anybody has mentioned this, but have you thought about using Time::Local? It is a core module ( bonus for portability ) and, to quote the manpage:

  These routines are the inverse of built-in perl fuctions
  localtime() and gmtime().  They accept a date as a six-
  element array, and return the corresponding time(2) value
  in seconds since the Epoch (Midnight, January 1, 1970).
  This value can be positive or negative.

I haven't done any bench marking, but I use these frequently to translate a human readable date ( like those found in syslogs and the like ) into seconds since the epoch. That makes it very easy to compare two dates, to calculate the delta between two dates, etc., without having to worry about month boundaries, leap years and other items that I almost always get wrong anyway.

If I have read what you are trying to do correctly, I think this module will do what you want.

mikfire