Well noe Captain Airclues, thanks for the technical description of a depaeture time. I realise that as the Number One of the craft you have to deal with so many different variables to depart on time so please let this PAX give his definition of "What a departure time actually means".
In my experience, it means anything between actually taking off, i.e. in the air at the time we are due to be in the air, ( yes lucky me, and the rest of the passengers) and 29 hours late via Virgin Atlantic, LGW-BOS with no apology, no food, no drink and 14 Hours on the M-25 motorway between LGW and Stanstead and back, arrival at Newark, with no luggage, and about 3 hours sleep, 200+ annoyed and bitter Americans! An Indian family, emigrating to the USA, under an armed police guard, because they apparently had no visa for the UK! Oh and yes, no compensation. A better take off experience was on Sudan Airways from Port Sudan to Jeddah! 18 hours late arrival at Jeddah via a ferry across the Red Sea!
Hey! you cannot blame the airline for the fact that they had no aircraft available!