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,