Work out flight duration across time zones instantly with clear inputs, formula shown and shareable results.
Flight duration must be computed in UTC, because local clocks change en route. Departing 21:30 at UTC+1 is 20:30 UTC; arriving 06:15 at UTC+8 is 22:15 UTC the same day, so the flight is 1.75 hours - whereas the clock difference of 8.75 hours is mostly the seven zones crossed rather than time in the air.
Convert both to UTC
UTC = local time - offset
Duration
duration = arrival UTC - departure UTC, adding 24 hours if negative
Because eastbound flights gain clock hours and westbound lose them. Only the UTC difference reflects real elapsed time.
It happens crossing the date line westbound. Converting both times to UTC resolves it correctly.