Montre automatique à lunette rouge et noire graduée sur 24 heures, cadran noir et bracelet jubilé

What is a GMT watch, and why did pilots adopt it?

It is 9 am in Paris. Your brother in Montreal is still asleep; your colleague in Tokyo is finishing the day. To work out who to call, you count on your fingers, get it wrong by an hour and start again. Long-haul flight crews in the 1950s lived with this puzzle every day, and it was for them that a now cult watch was born: the GMT watch.

A GMT watch is a watch that shows two time zones at once. As well as the hour and minute hands, it has a fourth hand that goes round the dial once every 24 hours, and a bezel marked from 0 to 24. The usual hands give local time; the 24-hour hand, read against the bezel, gives the time back home or in any other city. Three letters, one extra hand, and no more muttered sums. Here is where the name comes from, how to read and set a GMT, and what sets it apart from a watch that simply has a 24-hour bezel.

GMT: three letters from a London observatory

GMT stands for Greenwich Mean Time. Strictly speaking, it is the mean solar time at the Royal Observatory in Greenwich, on the edge of London, counted from midnight. For a long time every town set its clocks by its own sun: noon came earlier in Strasbourg than in Brest.

The railways changed everything. To publish consistent timetables, British railway companies adopted Greenwich time as early as 1847, and in 1880 it became the legal time across Great Britain.

1884: the world votes, France abstains

In October 1884, twenty-six countries sent 41 delegates to Washington for the International Meridian Conference. The aim was to choose a single prime meridian for maps and timekeeping. Greenwich won by 22 votes to 1 (the Dominican Republic), with 2 abstentions: Brazil and France, which was attached to its own Paris meridian.

France held out for twenty-seven years. It only adopted Greenwich time in 1911, and with truly diplomatic elegance: the official wording spoke of "Paris mean time, retarded by 9 minutes and 21 seconds", the exact gap between the Paris sun and the Greenwich sun. The phrase survived until 1978, when France adopted the vocabulary of Coordinated Universal Time.

Because today the scientific reference is no longer GMT but UTC, Coordinated Universal Time, kept by atomic clocks. UTC officially replaced GMT as the international standard on 1 January 1972. In everyday life the two are interchangeable, to within less than a second, and the name GMT has stayed on the dials.

1954: a watch for long-haul crews

In the 1950s, long-haul crews crossed oceans, changing time zones faster than their body clocks could follow, and had to juggle departure time, arrival time and the reference time used for navigation.

The first production GMT watch was born of that need. Launched in 1954, the Rolex GMT-Master was designed with the American airline Pan Am for its pilots and navigators (the brand is cited for history; SmodParis is an independent workshop, not affiliated with that house). It added an independent hand that turns once in 24 hours, and a rotating bezel split into two colours: red for daytime hours, blue for night-time hours. At a glance, the crew member knew whether it was 3 in the afternoon or 3 in the morning back at base.

Four wall clocks set to the time in London, New York, Tokyo and Moscow
Before the GMT watch, you needed a wall of clocks to follow several time zones. Photo: jarmoluk, Pixabay.

How to read a GMT watch in three seconds

A GMT is read in two steps. First like any watch: the hour and minute hands tell you the time where you are. Then find the 24-hour hand, often coloured and tipped with an arrow, and read the number on the bezel it points to. That number is the time in your second zone, in 24-hour format.

ElementWhat it showsExample
Hour and minute handsLocal time, where you are9.00 am in Paris
24-hour handThe time in a second zone, read on the bezelIt points to 3: it is 3 am in Montreal
Bezel marked 0 to 24The scale for the 24-hour hand; turned, a third time zoneTurned by the right number of clicks, it gives Tokyo time
Bezel coloursDay and night in the second zoneDark section: it is night over there

The 24-hour format is no detail. An ordinary hour hand goes round twice a day: 3 o'clock can mean the middle of the night or the middle of the afternoon. The GMT hand goes round only once. If it points to 15, it is 3 pm, with no ambiguity.

Setting the 24-hour hand

The manual for Seiko's 4R34 calibre, an automatic GMT movement, describes the most common method. The crown has three positions. At the first click, you set the date and the 24-hour hand: turn the crown and the GMT hand moves on its own while the seconds hand keeps running. At the second click, you set local time, with the seconds hand stopped. Seiko insists on one point: do not change the date between 9 pm and 3 am, or the next day's date change may not go smoothly.

The bezel, for a third time zone

The rotating bezel hides a trick. If the 24-hour hand already shows your reference time, turning the bezel lets you read a third time zone under the same hand. Seiko gives the rule in its instructions: work out the difference between the zone shown by the hand and the one you want to read, then turn the bezel by that many one-hour steps, clockwise if the result is positive. The manual's example: a hand set to Japan time (UTC +9) and a reading wanted for Los Angeles (UTC −8), which means 17 steps clockwise. Then put the bezel's 24 marker back at 12 o'clock.

True GMT or 24-hour bezel: know the difference

The success of the GMT look has a consequence: many watches wear a two-colour 24-hour bezel without the hand that goes with it. That is the case with our GMT Mod: a three-hand automatic watch with date, hand-assembled in Paris, which takes up the two-colour bezel, black dial and jubilee bracelet of the great GMT watches.

That is not a flaw, it is a different choice. A three-hand watch is simpler to set, and the 24-hour bezel serves as a reference and a style statement. But if you need to read two time zones at a glance every day, look for the extra hand: that is what makes a GMT.

Type of watchHandsBezelReading a second time zone
Four-hand GMTHours, minutes, seconds, 24 hoursMarked 0 to 24Direct, at a glance
Watch with a 24-hour bezelHours, minutes, secondsMarked 0 to 24Mental arithmetic, the bezel as a guide
Dive watchHours, minutes, secondsMarked 0 to 60 minutesNone: the bezel measures elapsed time

The last row clears up a common mix-up. The bezel of a dive watch, like the one on our Omeiko, is marked over 60 minutes and turns only one way: it times a dive, a boiling egg or a parking meter, not another time of day.

Automatic watch with a red and black 24-hour bezel, black dial and steel jubilee bracelet
The GMT Mod in red and black: the two-colour 24-hour bezel, signature of the GMT style.

France, world champion of time zones

Here is a question that wins every dinner-party quiz: which country has the most time zones? Not Russia, not the United States. It is France, with 12 different time zones, 13 if you count its claim in Antarctica. From the Marquesas to New Caledonia, by way of Guadeloupe, Réunion and Saint-Pierre-et-Miquelon, the sun never quite sets on French time.

Family in Pointe-à-Pitre, a son studying in Montreal, friends in Nouméa: for many people in France a second time zone is nothing exotic, and the 24-hour hand quickly becomes the most useful hand on the watch.

The history of French time has had its turbulent moments, too. In the summer of 1940, the occupying authorities put the northern zone on German summer time (GMT +2), while the southern zone stayed on French summer time (GMT +1). Until May 1941, when Vichy fell into line to unify the railway timetables, crossing the demarcation line meant changing your watch. The railways, once again.

Red and blue, red and black: what the colours say

On the first GMT, red and blue were not decoration: red marked the daytime hours, blue the night-time hours. Enthusiasts then gave nicknames to each combination, often borrowed from soft drinks or comic-book characters, and those nicknames ended up naming the colourways themselves.

  • Red and blue: the historic day-and-night combination, the most recognisable.
  • Red and black: more restrained; the red warms a black dial without making it loud.
  • Blue and black: the discreet version, equally at home with a suit or at the weekend.
  • Green and black: rarer, for those who want a GMT that does not look like everyone else's.
  • All black: the tool-watch look, for those who love the GMT for its lines more than its colours.

Our GMT Mod comes in all five versions, at the same price of 350 euros. In the workshop we notice that the choice almost always follows what people usually wear: red and black for darker wardrobes, blue and black for light shirts. If you are still wondering how to wear it, read why we wear a watch on the left wrist, or find out why watches always show 10:10 in photos.

Frequently asked questions

What does GMT mean on a watch?

GMT stands for Greenwich Mean Time, the mean time at the Royal Observatory in Greenwich, long the world's time reference. On a watch, the term means a complication that shows a second time zone using a 24-hour hand and a bezel marked from 0 to 24.

How can you tell if a watch is a true GMT?

Count the hands in the centre of the dial. A true GMT has four: hours, minutes, seconds and a hand that turns once in 24 hours, often coloured. A three-hand watch with a 24-hour bezel takes up the style, but does not read a second time zone directly.

What is the rotating bezel on a GMT watch for?

It is the scale for the 24-hour hand and, turned by the number of hours of difference, it lets you read a third time zone under the same hand. Its two colours separate the daytime hours from the night-time hours.

What is the difference between GMT and UTC?

GMT is the mean solar time at Greenwich; UTC is Coordinated Universal Time, kept by atomic clocks. UTC replaced GMT as the international reference on 1 January 1972. In daily life the two differ by less than a second.

Is a GMT watch useful if you do not travel?

Yes, as soon as a relative, a client or a team lives in another time zone. The 24-hour hand tells you instantly whether it is day or night over there, which saves you from calls at 3 in the morning. And the two-colour bezel is a pleasure to wear every day.

Sources

Further reading: what is a Seiko mod? and the movements in our watches.

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