Departure is defined as one of the following:
Actual Departure Time – ATD: Gate departure time is the instance when the pilot releases the aircraft parking brake after passengers have loaded and aircraft doors have been closed.
In cases where the flight returned to the departure gate before wheels-off time and departed a second time, report the last gate departure time before wheels-off time.
In cases of an air return, report the last gate departure time before the gate return.? If passengers were boarded without the parking brake being set, record the time that the passenger door was closed.
Also, carriers using a Docking Guidance System may record the official gate-departure time based on aircraft movement. For example, one DGS records gate departure time when the aircraft moves more than 1 meter from the appropriate parking mark within 15 seconds. Fifteen seconds is then subtracted from the recorded time to obtain the appropriate out time.