The industry usually refers to the off-block time as the published "departure" time, i. e. on a gate position requiring push-back it's the time the aircraft is pushed-back. On a stand without a pushback it is the time of the movement of the plane from the stand.
Of course there are some variations by different airlines, for example (my knowledge):
KLM Cityhopper: Time of Switching Anti-Collision Light on
Delta/Northwest: Releasing Brakes on Stand
No airline refers the Departure time to the actual "departure" (airborne) time.
Arrival? Well, it's the on block time at the stand/gate, not the "landing" time.
Greets,
Trabbi