As any good lawyer will tell you "it all depends".
If you are recording times for regulatory purposes such as pilot, airframe or engine hours for pilot qualifications and times between overhaul or for component lifeing, there are specific regulations set out for each case by the authorities (and plenty of people willing to argue over the interpretation
).
If you are an airline advertising how long it takes to get from A - B, it most likely is from pushback to brakes on but an airline is free to use any measure it likes as long as it isn't grossly misrepresenting the position.
DGG
