The OP's Q. is a good and tricky one. Actually FlightAware pulls data from the FAA's en route radar tracking system and that system does not track a plane until "wheels-up". However, as others correctly noted, the airline industry defines departure time as the time the a/c will "push" from the gate or stand. I verified this by comparing the data on two flights I took yesterday. The airline website data clocked departure times as when the planes left the gate. FlightAware clocked as the time the flights took off from the runway.