Flights are booked out just before the start of the flight. Pilots record the aircraft, persons on board and the estimated departure and return times. If no flight details are added to book the flight back in, the system generates a notification to the person who booked the flight out. This reminder is sent some time after the estimated return time, configurable in settings.
To allow for cases where there is no network connection at the airfield, pilots can book out early up to a maximum number of hours you specify in Maximum hours early. The estimated time of departure should be changed to the best estimate of when the flight will commence.
To allow for cases where pilots may have forgotten to book out, you can allow them to book out for a time in the past. Pilots can then set the actual departure time up to your setting for Maximum hours late. It is of course not recommended, but the setting allows for mistakes to be made. You may wish to enforce a more strict setting of zero.