I’m never able to figure out what is meant by the departure time. If a flight is due to depart at 1.30pm for example, does that mean that it will push back at 1.30pm, or should it be on the runway at 1.30pm, ready for takeoff?

As for a 20 minute delay… when you consider that an hour is assigned for such a short flight, a delayed departure doesn’t really matter. When the flight is complete in 40 minutes or so anyway, you only arrive one or two minutes behind schedule.

Departure time is when you push off the gate.