When editing Control Tower Hours field you can use two input formats:
Sunset/Sunrise format allows to input Control Tower Hours based on time (in hours-h or minutes-m) before/after Sunset or Sunrise.
For example if an airport opens 30 minutes after sunrise and closes 2 hours before Sunset, its opening time in Control Tower Hours field would be SR+30m and its closing time - SS-2h. Example of input in screenshot below
When the flight is scheduled to take off or land outside permitted hours the folloring message will display: Departure (Destination) airport is closed at the time of departure (arrival)