my petsitter is supposed to leave at 8 pm.. and we got stuck in traffic or some other problem..... what happens if we can't get home til 3 a.m.?
Everyone is subject to unexpected delays and many sitters will be quite understanding about it. I don't know if the example you gave has actually happened, but I can't imagine a 7-hour traffic delay. That being said, surely you don't expect the sitter to finish his/her job in the middle of the night? Either establish a more reasonable timeframe, let's say, 10 pm for the sitter to depart, if agreeable. Your dog would normally be sleeping during that time anyway. If you are worried about the house being unoccupied, then that's another story.
As Jennifer advises, discuss this with your sitter. A 7-hour overage deserves additional compensation and your sitter will ultimately decide how to handle it.