Date manipulation function

I have a website which displays a list of events and the dates they are occuring. These event reoccur at set intervals. I need someone to write a function to calculate the date of the next event and to check that the displayed date is not in the past. The intervals are: Weekly Fortnightly 1st week of every month 2nd week of every month 3rd week of every month 4th week of every month Last week of every month 1st & 3rd week of every month 2nd & 4th week of every month I have already created some of the function. I just need someone to complete it.