How a GMT watch works

A GMT watch adds a fourth hand that goes around the dial once every 24 hours instead of every 12. It points at a 24-hour scale, printed either on the bezel or around the dial’s edge. That hand can show a second time zone — and, because it covers the whole day, it always tells you whether it is day or night there.

Setting a second time zone

1. 1Set the main hands to your local time as usual.
2. 2Use the GMT-hand setting position to point the 24-hour hand at the hour in your second zone — in 24-hour format, e.g. 15 rather than 3.
3. 3If you have a rotating 24-hour bezel, start with its 24 marker at 12 o’clock.

Reading a third zone with the bezel

Rotate the 24-hour bezel by the time difference of a third zone. The GMT hand, now read against the shifted bezel, shows that third time. Try it below: the GMT hand is fixed at 14:00, and you turn the bezel.