The GMT function is a particularly useful complication for frequent travelers, as it allows them to quickly read the local and home time at a glance. A pioneer, and probably the best-known model in this genre, is the Rolex GMT-Master. But what does GMT actually mean and how do you use the function?
What is GMT time?
Greenwich Mean Time – formerly known as Universal Time – is a time zone that was defined at the International Meridian Conference in Washington in 1884. Associated with the Earth’s Prime Meridian, GMT was determined using measurements of the Earth’s rotation. The Prime Meridian runs from the North Pole to the South Pole and crosses the Old Royal Observatory in the London suburb of Greenwich. GMT originally served as a global standard time to which all local times around the world were to be oriented. One of the aims was to ensure the punctuality of trains. At that time, station clocks were based on the position of the sun, which was different at each location. As you can imagine, this led to inaccuracies and resulting delays. Shipping and air traffic also benefited from the introduction of GMT time. The GMT function played a particularly important role in early transatlantic flights in the 1950s, but more on that later. GMT was based on the position of the sun for 88 years until it was changed to UTC (Coordinated Universal Time) in 1972. UTC is determined by atomic clocks and is therefore considered far more accurate. Coordinated Universal Time is not a time zone like GMT, but a time standard for calculating local times worldwide. Some watch manufacturers such as IWC and Sinn use the designation UTC instead of GMT to describe their watches with a second time zone, but the functions are identical.
How do I use the GMT function?
Standard wristwatches generally measure a time interval of 12 hours. This means that the hour hand circles the dial twice within 24 hours. A GMT watch has an additional hour hand, which is typically arrow-shaped and has a different color than the 12-hour hand. In contrast to the hour hand, the GMT hand circles the dial once every 24 hours, i.e., half as fast. GMT time is usually read via a 24-hour bezel or rehaut. In addition to their unique aesthetics, watches with a GMT function are particularly practical for those who travel internationally. If you are flying from New York to London tomorrow, you are probably aware that there is a time difference of 5 hours. When you arrive in London, you always have to calculate to figure out the time back home. It gets even more complicated if you also have relatives in Hong Kong. A GMT clock solves this problem in a very simple way. Here’s an example of how to use the GMT function on a watch with a separate hour hand. This is the set-up on models such as the Tudor Black Bay GMT with the in-house MT5642 movement or the Rolex GMT-Master with caliber 3258. Before you leave or when you land in London at 20:00 local time, set the GMT hand to your home time (15:00). Then set the hour hand to the local time in London (20:00). So that you also know what time it is in Hong Kong, move the 24-hour bezel to the corresponding local time (04:00) in the same way as the GMT hand. With just a few simple steps, you now have three time zones in view at the same time.
When Time Learned to Fly
As mentioned at the beginning, the Rolex GMT-Master is probably the best-known and most characteristic GMT watch. Rolex launched the first variant on behalf of the former Pan Am airline in 1955. The early 1950s marked the advent of the first transatlantic flights and people were suddenly able to cross oceans and time zones in one day. Rolex was commissioned by Pan Am to design a new type of pilot’s watch for its crew. The watch not only had to be easy to read, but also have an additional time zone. The result was the Rolex GMT-Master with the reference number 6542. The GMT-Master and later GMT-Master II have been successful additions to the Rolex catalog since 1955. Over the years, it has undergone very thoughtful updating, making it the unmistakable watch it is today. The GMT-Master may be the most famous watch with a GMT function, but it is not the first. The Glycine Airman first saw the light of day back in 1953. In 1953, Samuel W. Glur – then a salesman at Glycine SA – was talking to pilots on one of his many flights about what the ideal pilot’s watch might look like. The answer was that the ideal watch should have a second time zone and also be waterproof. As a clever businessman, Glur acted immediately and forwarded the idea to Glycine in Switzerland. In that same year, the developers presented one of the first world timepieces ever with the Airman – even before Rolex. Today, the Airman remains one of the most popular models with a GMT function among watch enthusiasts.
Differences Among GMT Functions
The first GMT-Master with the reference number 6542 was powered by the caliber 1036. Here, the GMT hand moves in tandem with the hour hand. This means that the hands on this movement cannot be set independently of each other. The second time zone is therefore displayed exclusively via the bidirectional bezel with a 24-hour scale. It was not until the mid-1980s that Rolex introduced the caliber 3085 , a movement in which the hour hand could be set independently of the GMT hand. Another well-known example of this design is the Omega caliber 1128, as used in the Seamaster 300 GMT . Seiko also offers a movement with this function with their caliber 9S86 . It is used in the Grand Seiko models SBGM227 and SGBJ203, among others.
Well-Known Watches With a GMT Function
In addition to the movements and variants described above, there are many other manufacturers and models that display a second time zone in different ways.
1. Omega Seamaster Planet Ocean GMT
The Omega Seamaster Planet Ocean is a popular GMT watch. The three-hand watch is powered by the chronometer-certified, in-house Omega caliber 8906, which has a separately adjustable hour hand. The GMT time is read in the classic way using the 24-hour scale on the ceramic bezel. With a diameter of 43.5 mm, the Planet Ocean is one of the larger watches in its class and is correspondingly eye-catching. The water resistance is up to 600 m (60 bar), which makes it not only an attractive GMT watch, but also a genuine diver. Pre-owned, this model can be found in very good condition from around $2,800 on Chrono24.
2. Longines Spirit Zulu Time Titanium
A lightweight in this list is the Longines Spirit Zulu Time Titanium. With its 39-mm case and matching titanium bracelet, it is not bulky on the wrist and attracts attention with its timeless and vintage-inspired looks. At the heart of the timepiece ticks the in-house caliber L844.4, which drives one hand each for hours and minutes as well as the red-accented 24-hour GMT hand. The movement has a power reserve of 72 hours. A bidirectional anthracite ceramic bezel frames the same-colored dial with Arabic indices and a date window at 6 o’clock. All in all, this is a featherweight companion with understated retro charm that could be yours from a price of $3,500.
3. Tudor Black Bay 58 GMT
Launched in 2024, the Tudor Black Bay 58 GMT is part of the popular Black Bay 58 series. Just like the Longines Spirit Zulu Time, it comes with a unisex 39-mm case and a bidirectional rotating bezel with a 24-hour scale – here in black and Bordeaux. The black dial with gold-colored accents provides the classic retro aesthetic for which the Black Bay 58 models are known and loved. The watch is powered by the in-house MT5450-U movement, which offers COSC-certified chronometer accuracy and a power reserve of 70 hours. The watch is water-resistant to 200 m (20 bar) and is offered with various strap options. For this classic on a three-piece link stainless steel bracelet with a “T-Fit” clasp, prices on Chrono24 start at just under $4,200 for a pre-owned example.
