Think about it this way: Assume you live in L.A. and were going to take the flight you're worried about. You check in online several hours ahead of your flight and note that the plane is still scheduled to leave at the same time you expect it to leave. You arrive at the airport on time only to find the plane has already left well ahead of schedule. Does this ever happen? Not that I'm aware of. I have had the flight canceled in this window, but I'm then rebooked on a later flight, never on an earlier one.