scheduler algorithm

hi... i'm creating a quick/dirty app for scheduling a function, based on the user entering a start time/date, as well as a potential periodic timeframe. ie: the user enters- start time/date: 10:00am 01/10/07 periodic monthly the idea is to be able to have a process start running on the start time/date, and then to repeat on a monthly basis on the same time/date. i can easily accomplish this for dates that are '1-28', but i'm looking for suggestions as to how to handle situations where the number of the start date, might be greater than the end date of the 'current" month. in searching google, i haven't come up with many apps that handle this... thoughts/comments/etc... thanks. -bruce