Most regulatory bodies would probably say it is from the time the aircraft begins to move under it's own power until the aircraft is stopped at the gate and the engines shut down.
This issue often arises because the ACARS OOOI times are generally linked to different parameters and (typically) relate to last door closed time for the commencement of the flight ("OUT" time) and park brake set for arrival ("IN" time).
Many companies electronically now use ACARS times as valid flight crew records. Seems eminently practical.