Whether the watch is a traveler's GMT or a caller's GMT, the GMT hand moves in tandem with the hour hand, only at half the speed. When it is set to a different time zone, it indicates a foreign time of your choice.

GMT watches feature an additional 24-hour hand, so they show you two-time zones at once (sometimes three, if there is also a 24-hour bezel). While a normal hour hand travels around the dial once every 12 hours, the GMT hand travels around the dial only once every 24 hours.

Although both variations are GMT watches, the “true GMT” name typically refers to timepieces where the 12-hour hand can be adjusted independently, while the “office GMT” moniker describes those with an independently adjustable 24-hour hand.