GMT

A GMT watch shows a second time zone using an extra hand that makes one revolution every 24 hours instead of every 12. Read against a 24-hour scale, printed on the dial's inner ring or on the bezel, that hand tells you the time somewhere else without ambiguity between morning and evening. That is the whole point of the format: an ordinary hour hand cannot distinguish 3am from 3pm, which is exactly the fact you need before telephoning anyone.

Where it came from

The name comes from Greenwich Mean Time, mean solar time on the prime meridian, which is what aviation used as its common reference when such watches reached the mainstream in the 1950s. Rolex's GMT-Master was developed with Pan American World Airways for crews crossing several zones a day, and the category kept the name. The modern civil standard is UTC, an atomic timescale held close to the earth's rotation. For anything a wristwatch does the two are the same, differing by well under a second, which is why some brands print UTC on the dial and mean nothing different by it.

The other date worth knowing is the early 1980s, when Rolex gave the GMT-Master II an hour hand that could be moved independently. That single change created the split that still decides which GMT you should buy.

The two kinds

The distinction that matters when buying is which hand you can adjust independently.

In a caller GMT, sometimes described as an office GMT, the 24-hour hand is set independently while the main hour hand runs continuously. This suits someone who stays put and calls another time zone. The date has its own quickset, unconnected to the 24-hour hand. The ETA 2893-2 and Seiko's NH34, the GMT member of the NH35 family, both work this way.

In a traveler GMT, often called a flyer or true GMT, the local hour hand jumps forward and back in one-hour steps without disturbing the minutes, seconds or the 24-hour hand, and the date follows it across midnight in either direction. This suits someone who lands in a new country and wants local time reset in a few clicks while home time stays untouched. Rolex's caliber 3285 and the Miyota 9075 are examples.

Neither is better in the abstract. They are built for different users, the specification sheet frequently fails to say which a watch has, and the traveler costs more to build because it needs an independently indexed hour wheel with its own jumper. If a listing says only "GMT function", assume caller.

Reading a third zone

If the 24-hour scale sits on a bidirectional rotating bezel, you can rotate it to offset the GMT hand and read a third time zone. Most such bezels click in 24 positions, one per hour, 15 degrees of rotation each. Rotate by the difference between the zone the hand is set to and the zone you want, then read the same hand against the moved scale. That is the traditional pilot's trick, and it is why so many GMT watches pair a 24-hour hand with a turning bezel rather than a fixed one.

It also explains a design rule that confuses people: a GMT bezel turns both ways, and a dive bezel must not. ISO 6425 requires a dive bezel to be unidirectional so that a knock can never make your remaining air look longer than it is. A GMT bezel is an information display; a dive bezel is a safety device.

What it cannot do

A GMT hand indexed in whole hours cannot show a zone that is offset by part of an hour. India is UTC+05:30, Nepal is UTC+05:45 and the Chatham Islands are UTC+12:45, and none of them can be read off an hour-stepped hand. A friction-fitted bezel, which stops anywhere rather than in 24 clicks, is the usual workaround.

Nothing in the watch knows about daylight saving either. That is legislation, it moves on different dates in different countries, and it means a manual correction at each end of the year.

Common misconceptions

A GMT hand is not a world timer. A world timer displays many zones at once, usually around a rotating 24-hour ring read against a list of reference cities. A GMT watch shows one extra zone, or two with a rotating bezel, and nothing more. The two are set out side by side in GMT vs world timer.

A 24-hour hand is also not automatically a GMT hand. Some watches carry a 24-hour hand geared straight to local time and not adjustable at all; that is a day and night indicator, useful on a dark dial and useless as a second zone. Check that the hand can be moved on its own before you count the watch as a GMT.

Finally, "dual time" is a looser term. It covers GMT watches but also watches showing the second zone on a 12-hour subdial, which gives you the hour and drops the information about whether it is day or night there. Our guide to GMT and world time watches goes through each layout, and what is a GMT watch for covers whether you need one at all.