But the other evidence, waypoints, comms being switched and etc, tell us that someone with significant knowledge was flying the aircraft. They have the passenger manifest and should be able to determine if there was anyone else besides the pilots with significant flying ability. Apparently they have the voice recording of the final message and are analyzing it for signs of stress. So knowing the precise timing is going to provide a minor extra clue that is of minor assistance to finding the aircraft, given the information they already know.
Not to mention they can't possibly know the exact timing until they find the aircraft. The thing transmitted but then didn't transmit as expected 30 minutes later. Exactly how can they determine when it was turned off without the aircraft? Media beat up.