First of all, PrimeGrid is unlike some other projects in that we have always permitted computers to return late tasks and still get credit, providing, of course, that the result is correct.
PrimeGrid is unlike ALL other projects (I think) in that we have a system that automatically extends the deadline as long as your computer is actually working on the task and communicating this to the server (which it should do once per day.)
Please read this thread for more details.
Please do not PM me with support questions. They will usually go unanswered. Ask on the forums instead. Thank you!