Arrival time is when the parking brake is set by the flt crew (prior to jetway being connected, just after aircraft stops moving and is chocked by the ramper).

Departure time is when the parking brake is released.

Notice all times are dependent on the parking brake -- this is significant since one can release the parking brake and depress the normal brakes (just like on a car) without the aircraft moving to get an out time. Once released, the parking brake can even be reset without changing the previously reported time. Few flight crews are willing to do this, since there is a slight chance the aircraft will move in the process (an FAA violation) and do to the fact that its simply unethical.

joe