There are free tutorials relating to using JavaScript to do time/date calculations. Here are more links by the same author:
http://acrobatusers.com/tutorials/date_time_part1
http://acrobatusers.com/tutorials/working-with-date-and-time-in-acroba t-javascript-part-2
http://acrobatusers.com/tutorials/working-date-and-time-acrobat-javasc ript-part-3-3