Date Problem

Hey Guys, I am working on a project for a client who is a retailer of inverters and batteries, and currently the system records the salesdetails including the warranty expiry date which is always a year(12 months). But he wishes to provide support and service every 3 months and that seems to be the beginning of my problems :D if the warranty on billno ='100' expires on 2012-09-27, the system has to show him this billno 4 times as a service due. Eg: a product was sold and a bill generated on 2011-9-27, the warranty on the products expires on 2012-9-27, so the system has to show the first service due on 2011-12-27, then one on 2012-3-27, then on 2012-6-27. How do i go about it? Any help is much appreciated. Thanks,