we tell any pax who may be late to the gate for what ever reason and who have been denied boarding that the scheduled time of departure on their boarding pass is the time it HAS to be in the air by
but seriously where I work and the airlines we handle, and listening very carefully to the pre-flight announcement by the flight deck prior to departure and en-route and arrival, statistics wise it is classed as the on/off blocks time.
This is certainly how the UK's largest airline measures their OTP!