A GMT watch shows the time in two time zones at once. As well as the usual hands, it has an extra hand that runs on a 24-hour cycle and points to the time in a second zone. Frequent flyers value it because they need not work out the time elsewhere in their heads.
How does a GMT watch work?
A GMT watch has two hour hands. The normal hour hand goes round the dial twice a day. It shows local time, where the wearer is. The GMT hand goes round only once a day, so it reads against a scale of 24 hours rather than 12. It shows the time in a second zone. The wearer reads both times at the same moment, from the same dial, with no sums to do.
What is the 24-hour bezel for?
Some GMT watches have a turning bezel, the ring around the dial, marked with 24 hours. The wearer turns it to line it up against the GMT hand, and in this way sets the second zone. The bezel is found on some models, not on every GMT watch. Where there is none, the GMT hand alone shows the second zone.
Where it comes from
The name comes from Greenwich Mean Time, the reference time set in 1884 and tied to the Royal Observatory in London. GMT watches first took off in the Jet Age, when commercial airlines began flying overseas. Pan Am’s senior navigator, Captain Frederick Libby, was given the job of picking a company to make a watch that always showed GMT on transoceanic flights. The Rolex GMT-Master was made together with Pan Am and came out in 1954. The first GMT-Master, reference 6542, had its own separate 24-hour hand. In 1953 the Glycine Airman came out, designed with input from the US Air Force.
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