If you ask before the due date I will almost certainly say yes, so just ask! If the due date has passed, the answer will be no.
I greatly appreciate the way this is set up. I feel that it gives enough flexibility that if we are responsible and looking ahead to our schedule and feel as though we might not be able to do the best quality work, we can ask for more time. This does a good job of holding us accountable while at the same time lending us some help when we may need it.