Most of the other posts here are using Unix timestamps to do the date arithmetic, and it is a good idea for this particular case (especially when Perl comes with the functions needed for it). It is not a good idea to write the millionth instance of manual date arithmetic code in existence, when the vast majority of them are riddled with bugs. (See: recent outages involving 29 February)

(I generally only trust well-known modules or an SQL database if I need to do date arithmetic)