Local times in, real durations out
A flight leaving New York at 21:00 and landing in London at 09:00 the next day takes seven hours, not twelve: London clocks are five hours ahead. Enter each departure and arrival exactly as printed, with the airport’s zone, and the calculator converts both ends to instants before subtracting.
It also checks chronology: an arrival before its departure, or a connection that leaves before you land, usually means a date was typed wrong. Connections shorter than the minimum you set are highlighted. No flight data is looked up and nothing is guaranteed about real schedules.