Sure...I guess when I hear "time bomb" I think of something that wipes source code or data. Something that alters the bits and bytes. That's the thing that will cause you a world of hurt. What you're talking about is of course totally legit.
I sold a piece of software to a local business a few months ago that works on expiring dates. Each payment I get from the owner pushes back the expiration date until I get the full amount, at which point it's theirs forever(it's worth mentioning that so far things are great).
We didn't agree to a contract for the work I did, so I can only hope I don't get screwed over some how in the future. It was my first freelance job though, so I guess there's a lesson to be learned.