Formula for calculate date after adding 90 days to it

What formula can I use to calculate a Date2 based on another Date1. For instance Date1 would be the commencement date, and Date2 will be a Disbursment date. Date2 needs to be calculated based on Date1 + 90 days.

For example, if DATE1 ( Commencement Date) = Sept 1/05 and it is a seven month program, then DATE2 should be 1st day of 4th month, i.e. Dec 1/05

if nine month program, then DATE2 will be made on 1st day of fifth month, i.e. Jan 1/06

Any help in this regard much appreciated.

thanks,

Thank you that was an awesome pointer.
-aman