Also: FDP, Crew duty time, Crew-Dienstzeit
In Europe, EASA flight and duty time rules set how long a crew may work in commercial operations and what rest periods must follow. The permitted length depends, among other things, on the time duty starts and the number of sectors flown.
This has practical consequences for private jet travel. A day with positioning, an outbound flight, a long wait and a return flight can exceed the permitted duty time. A second crew or an overnight stay is then required.
Delays matter too: if a departure slips significantly, the crew may run out of duty time. On day trips, plan realistic time on the ground.