If a train reaches its scheduled stop on-time, then it can leave the stop at its scheduled departure time (if the signal aspect is GREEN)
If a train is early, it should wait until the scheduled departure time to depart. This is because passengers plan their arrival at the station based on the scheduled departure time.
If a train arrives beyond its scheduled arrival time, then it must stop for the duration of the scheduled stop. The “duration” is defined as the time difference between the scheduled departure & arrival times for the train at that station.