Oct 29, 2019
 6 minutes

What is a GMT function and how does it work?

By Sebastian Swart
What is a GMT function and how does it work?
What is a GMT function and how does it work? Image: Bert Buijsrogge

A GMT function is a particularly useful complication for frequent travelers because it allows you to track two time zones simultaneously. One of the most famous watches with this feature is the Rolex GMT-Master. But what does GMT stand for and how does the complication work?

GMT: Greenwich Mean Time

Greenwich Mean Time (GMT) is the time associated with the prime meridian, which runs through Greenwich, England. This point was chosen as Earth’s zero degrees of longitude at the International Meridian Conference held in Washington, D.C. in 1884. Most time zones are based on GMT, i.e., number of hours ahead of or behind GMT. The Greenwich Meridian runs from the North Pole to the South Pole, crossing through the Old Royal Observatory in South East London’s Borough of Greenwich.

GMT initially served as an international standard time, around which most other local times were based. The move to standardize local times was in large part driven by a desire to regulate train schedules. At the time, each individual railway station based its clock on the position of the sun, which varies by location. As you can imagine, delays and confusion were very common. GMT time also played a significant role in maritime navigation and later, in transatlantic flights.

For 88 years, GMT was determined by the position of the sun. In 1972, however, GMT was superseded by Coordinated Universal Time (UTC), a more stable time which is based on a series of atomic clocks. Universal Time is not a time zone in itself but rather a standard of reference for every location around the globe. Several manufacturers, including IWC and Sinn, use the abbreviation UTC instead of GMT for their watches with a second time zone, though the features are completely identical.

How do GMT complications work?

Rolex GMT Master
Rolex GMT Master

Most standard watches feature a 12-hour time interval on their dials, meaning the hour hand will travel around the dial twice in each 24-hour period. If the watch has a GMT function, it typically features an additional hour hand of a different shape and color. While the 12-hour hand completes two rotations each day, a GMT hour hand usually travels just once around the dial in a 24-hour period and references a 24-hour bezel or ring around the edge of the dial.

GMT watches certainly have a unique look. However, more importantly, they are practical, especially if you’re traveling between time zones. If you’ve flown from New York to London, for example, you know that the UK is 5 hours ahead. Once you arrive in London, you constantly have to calculate what time it is back home if you don’t have a GMT watch. Things get a bit more complex if you also have colleagues or family on the West Coast as well. A GMT watch is the perfect solution if you find yourself in this situation.

Models, such as the Tudor Black Bay GMT with the in-house caliber MT5642 or the Rolex GMT-Master with the caliber 3258, have an independently adjustable hour hand, meaning once you land in London, you can set the GMT hand to your home time.You can then set the primary hour hand to the local time in London. You can also keep track of the time on the West Coast by moving the 24-hour bezel to the appropriate time. Thus, with just a few minor adjustments, you can suddenly keep track of three time zones at once.

Time Flies

Rolex GMT Master
Rolex GMT Master

 

As mentioned, the Rolex GMT-Master is one of the most iconic and well-known GMT watches on the market. Rolex created this watch for the former airline Pan Am in 1955. The early 1950s were marked by the advent of commercial transatlantic flights. People were suddenly able to cross oceans and time zones in a single day. Pan Am commissioned Rolex to develop a new watch for their pilots. The watch had to be easy to read and feature an additional time zone. Thus, the first Rolex GMT-Master was born, with the reference number 6542. The Rolex GMT-Master, and later the GMT-Master II, have been successful members of the Rolex catalog ever since. The models and their features have been carefully adjusted since 1955, making them irreplaceable Rolex classics.

While the GMT-Master is one of the more famous GMT watches, it isn’t actually the first. In 1953, the Glycine Airman entered the market. Earlier that year, Samuel W. Glur, a salesperson with Glycine SA, consulted with numerous pilots to find out what an ideal pilot’s watch might look like. Two features were deemed essential: water resistance and the ability to keep track of a second time zone. Glur, ever the businessman, brought his findings back to Switzerland. Glycine’s engineers managed to present the Airman that very same year, ahead of the Rolex GMT-Master’s debut. Today, the Airman remains one of the more popular GMT watches.

Characteristic GMT Features

The first Rolex GMT-Master, ref. 6542, is powered by the caliber 1036. On this watch, the GMT hand moves in tandem with the main hour hand, meaning the hands cannot be set independently of one another. The second time zone can only be displayed via the bidirectional rotating bezel with a 24-hour scale. In the mid-1980s Rolex released the caliber 3085, which was the first movement with independent hour and GMT hands. Another movement with this design is the Omega caliber 1128, which appears in the Seamaster 300 GMT. Seiko offers a similar function with their caliber 9S86. This movement powers several Grand Seiko models, including the SBGM227 and SGBJ203.

Omega Seamaster GMT-4787
Omega Seamaster GMT-4787, Image: Bert Buijsrogge

 

The Most Popular GMT Movements

Developing and producing an in-house caliber is complex and expensive. Not all watch manufacturers can afford the time and expense required to create their own GMT movements: Enter the tried and tested Swiss movement suppliers. The most frequently-used ébauche GMT movements are the ETA 2893-2and Sellita SW 330-1, both of which offer a relatively economical entry into the world of GMT calibers. These movements allow you to set the GMT hand, but not the hour hand. ETA movements are frequently used in watches by brands in the Swatch Group. Some well-known models include the Longines Conquest V.H.P. GMTand the Union Glashütte Belisar GMT. Today’s Glycine Airman models use Sellita movements, but change their names, e.g., GL293.

In 2002, Glycine launched the Airman 7, a truly unique watch that displays four time zones. Glycine utilizes three mechanical calibers that run simultaneously across three different dials. This model is innovative both in terms of technology and design, and it certainly stands out with its unique look.

A Selection of Well-Known GMT Watches

There are a number of other manufacturers and models that also display two time zones via creative means. Some Panerai models manage to display the entire GMT function on the dial; for example, the Panerai Luminor 8 Days GMT ref. PAM00233 with the in-house P.2002 movement. This 44-mm watch features its second time zone at 9 o’clock. It also has an 8-day power reserve and is water-resistant to 10 bar (100 m, 328 ft).

Omega Seamaster Planet Ocean GMT

The Omega Seamaster Planet Ocean is another beloved GMT model. This three-hand watch is powered by the in-house, chronometer-certified caliber 8906with an independently adjustable hour hand. The second time zone is displayed with a classic 24-hour scale around the ceramic bezel. At 43.5 mm, the Planet Ocean is one of the larger and more imposing GMT models available. It is water-resistant to 600 m (60 bar, 1,969 ft), making it both an appealing GMT watch as well as a professional-grade diver.

Tudor Black Bay GMT
Tudor Black Bay GMT

Tudor Black Bay GMT

Tudor introduced the Black Bay GMT to great acclaim at Baselworld 2018. The watch debuted alongside the new Rolex GMT-Master II “Pepsi,” and the two timepieces have a number of aesthetic touches in common, including the characteristic red/blue bezel and red GMT hand. The in-house, chronometer-certified Tudor MT5652 movement powers the Black Bay. The bidirectional, stainless steel bezel features an aluminum inlay. This watch is 41 mm in diameter and water-resistant to 200 m (20 bar, 656 ft). The Black Bay GMT is available on a stainless steel bracelet (ref. 79830RB) as well as a leather or NATO strap.

Read more

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What is a tourbillon and how does it work?

What is an in-house movement?


About the Author

Sebastian Swart

I've been using Chrono24 for years to buy and sell watches, as well as for research purposes. I've had an infatuation with watches for as long as I can remember. As a …

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