Attempting to patent a non-extant invention would seem unwise. If the patent became granted, I think it would start the 15-year clock. After that timeout, you l

Attempting to patent a non-extant invention would seem unwise. If the patent became granted, I think it would start the 15-year clock. After that timeout, you lose the patent. If you actually create the invention some time after somehow already being granted its patent, you have less time to make use of your exclusive rights.