Time Zones and Elapsed Time

Duration Versus Clock Time

The time between two events is fixed regardless of where you observe it from. Two meetings three hours apart are three hours apart in London and in Tokyo. What changes is what the clock reads, which is what you enter into this calculator.

Convert Before Subtracting

To measure the elapsed time of a journey, convert both the departure and the arrival clock times into the same zone first, then subtract. Subtracting local times in two different zones gives a number with no meaning.

The Overnight Flight Problem

A flight leaving Tokyo at 11:00 and arriving Los Angeles at 03:00 the same day looks like a negative journey, and the subtraction gives minus 8 hours. Converted into a single zone it is a normal flight of about 11 hours. The raw clock difference was never a duration.

Daylight Saving Changes

Twice a year, an hour is inserted or removed, so a day can have 23 or 25 hours. A duration measured across a transition is genuinely different by an hour from the naive calculation, which matters for anything scheduled automatically.

Recurring Meetings Across Zones

When a meeting recurs weekly, the local time shifts by an hour either side of a daylight saving change. Fixing it in UTC and converting for display is the reliable approach, and it is why calendar invitations carry both a zone and an absolute time.