You log the time from the moment the airplane begins to move under its own
power for the purpose of flight. So the flight definitely starts before you
leave the airport. It starts before you take off for the first time.

On my second student cross-country, my door popped open right after
takeoff, and I had to circle around and land again, shut the door, and
take off again. Afterwards, I asked my instructor whether that first
trip around the pattern counted as part of the cross-country. He said
yes, because my intention (from the moment the engine started) was to
fly somewhere else. He said this logic also applies to the 20 minutes
that I waited in line for departure on the ground.

--Kath