I have been wondering, which time for departure/arrival is used to determine whether a plane is on time or not compared with timetables. Is it the time where the plane leaves the gate and arrives at the gate or is it the time where is takes off and lands? Or in other words, is a plane supposed to leave the gate at the time mentioned in the timetable or is it supposed to take off at that time? Similar in the other end, is it supposed to meet the gate at the time mentioned in the timetable or is it supposed to land at that time?