A flight leaves Dubai at 21:10 local time and lands in Delhi at 02:15 the next morning, Indian time. Indian time runs 1 hour 30 minutes ahead of Dubai time. How long was the flight?

The two clock readings are on different clocks, so one must be converted before subtracting. Landing at 02:15 Indian time is 00:45 by Dubai time, and from 21:10 to 00:45 is 3 hours 35 minutes. Subtracting the raw readings, 21:10 to 02:15, gives 5 hours 5 minutes and quietly credits the flight with the 1 hour 30 minutes the traveller gains on the clock. Converting the wrong way, to 03:45 Dubai time, gives 6 hours 35 minutes, and applying the correction a second time to the right answer leaves 2 hours 5 minutes.