I was wondering, what exactly is "departure time"?
Is it when the doors are shut, the aircraft pushes back or the aircraft leaves the ground etc?
"Departure time", as in STD, ATD, ETD, etc is the time the aircraft starts to move away from the gate, i.e. the start of the pushback.
Airlines typically record 4 timestamps for every flight:
Out = ATD as above
Off = airborne time
On = landing time
In = arrival at the gate (ATA)
usually referred to as the OOOI times.