A GMT watch shows two time zones at once: the time where you are and the time at home. The Fondation Haute Horlogerie notes that this function is very often paired with a 24-hour scale or a day/night indicator, so you can tell whether it is day or night on the other side of the world.
Local time, home time
Most often, local time, also called "travel time", is shown by the central hour and minute hands. Home time is read on a separate indication. According to the same source, most dual-time watches only add an hour indication for home time, because time differences are generally counted in whole hours.
A few models add a minute indication for the handful of countries offset by 15, 30 or 45 minutes from their zone.
Changing zone without upsetting the watch
When travelling, local time has to be reset at each move. To make this easier and spare the movement, some mechanisms, operated by the crown or by +/− pushers, move the hour hand alone, hour by hour, forwards or backwards (Fondation Haute Horlogerie). The minutes and seconds stay put: the watch keeps its time.
Four ways to show home time
The Foundation lists the most common solutions: an extra hour hand on the main dial, in a different colour; a subdial; an aperture; or a 24-hour scale on a rotating bezel or flange. In the last case, once local time is set, you turn the bezel to align home time with the hour hand.
The GMT-Master, born with the jet age
In the 1950s, Rolex recounts, airliners began crossing several time zones quickly. Knowing the time in several places at once became important. The GMT-Master was designed as a navigation aid for professionals and became the official watch of several airlines, including Pan American World Airways. Its two-tone bezel marked daytime from night-time hours. The Fondation Haute Horlogerie dates its launch to 1954 and describes it as the watch that prefigured all the GMT watches to come.
GMT or world time
The Foundation distinguishes GMT from world time, which permanently shows the time in 24 cities. A GMT follows two, sometimes three or four, zones chosen by its wearer.
Frequently asked questions
What does GMT stand for?
Greenwich Mean Time. The term has become the name of this complication.
Why a 24-hour scale?
To tell 8 in the morning from 8 in the evening in the other zone.
Further reading
The School's GMT module dissects the second time zone mechanism. Our article on the crown and its positions explains the settings, and the Décalibre School offers the full course.