Understanding the GMT Function: What It Is and How It Works

Complications make timepieces more interesting, and the GMT function is arguably one of the most interesting complications in the watch market. It’s why watches like the Rolex GMT always make the list of the most iconic timepieces in the market.

The design of a typical GMT watch, including the extra hour hand and a rotating (typically colorful) bezel, gives GMT timepieces a unique and highly recognizable appearance. On top of that, the GMT complication itself is very useful in everyday life. It’s even more useful for frequent travelers, international business people, pilots, and people with family abroad. Not to mention, they’re among the most accessible complications in the horological world in terms of price.

There are many great reasons to want to buy a GMT timepiece. However, despite their popularity, not everyone understands how they work. That’s probably because there are many different GMT mechanisms with slightly different ways of setting or checking the time. This post will explain the GMT function, exploring its history, components, and how it works in real life.

What is the GMT function?

The GMT function is a useful complication that makes it possible to track two time zones on one timepiece at the same time. This complication is useful for people who want to be able to tell the time in multiple locations simultaneously, such as frequent travelers, people who do business abroad, and people with family somewhere in a different time zone.

The Acronym GMT stands for Greenwich Mean Time. This time runs through Greenwich, England, a point on Earth’s zero degrees longitude. At the International Meridian Conference in Washington in 1884, the Greenwich Mean Time was selected as the international reference point for standardizing local time. Consequently, time zones in various locations are based on the number of hours ahead or behind the GMT.

Consequently, when watchmakers started developing timepieces that would allow pilots to tell the time in multiple time zones at a glance, the name GMT watches was naturally adopted for these timepieces.

History

The history of the GMT complication is tied to the advancement of transatlantic air travel. On May 2, 1952, a De Havilland Comet airplane owned by the British Overseas Airways Corporation (BOAC) flew nonstop from London to Johannesburg. This flight signaled the beginning of a new era in aviation where people could not fly across multiple time zones in a single flight.

Pilots and passengers traveling on these long-haul flights needed a way to track the local time at their destination and the time back home. This necessitated the creation of wristwatches capable of doing this. Hence, the GMT complication was born.

The creation of the GMT complication is credited to a small watch brand known as Glycine. The Swiss watch brand created the Airman travel watch in 1953 as the first watch capable of simultaneously tracking two 24-hour time zones.

A year later, Rolex introduced the timepiece that popularized the concept of dual time zone watches. The GMT-Master was released in 1954 based on a special request by Pan American Airways. This watch and its predecessor, the Rolex GMT-Master II, would go on to define this complication. They’re still considered a blueprint for watches in this niche.

In 1972, the GMT standard was replaced by the Coordinated Universal Time (UTC), a standard reference established based on a series of atomic clocks. As a result, many manufacturers now refer to their dual time zone watches as UTC timepieces instead of GMT. Despite this, GMT remains the most popular way to describe a watch with a second-time zone in the horological world. Other common names include Pilot time watches and Traveler's watches.

How Does a GMT watch work

Today, a GMT watch comes in many flavors. However, the core functionality in all instances is the ability to track an additional time zone. The original Rolex GMT had a standard 12-hour display to show the local time, a two-tone rotating bezel with a 24-hour scale, and a second-hour hand that allowed the wearer to track the time in another time zone.

GMT watches today rely on different displays to achieve this. Some watches have a 24-hour central hand, others have a sub-dial for the second time zone, while some models have an aperture-based display.

However, the general idea in all cases is to have a traditional 12-hour display for local time and an additional 24-hour scale for time spent abroad. Some models also have a 12-hour display with a day/night indicator (instead of the 24-hour scale). The bezel can be fixed or rotated, with a rotating bezel introducing a third-time zone into the mix. The most important thing in any instance is the design of the dial and how the watch is set.

For instance, if you fly from London to New York, your new location is 5 hours behind the London time. Instead of trying to calculate the time in your head each time you check or setting the watch based on your current time alone, a GMT timepiece makes things a lot simpler.

For many models, such as the Rolex GMT master watches with an independently adjustable hour hand, you can simply set the GMT hand to point to your home time (the time back in London) and set the primary hour hand to point to the local time (the New York). Now, when you check the time, you can see the time in both locations at a glance.

On most modern GMT watches (the ones with a rotatable bezel), you can even see the time in a third location with just a few adjustments to your watch. Say you want to track the time on the West Coast. You must move the rotating bezel by the number of hours for the new time zone you want to track.

Types of GMT watches?

As noted above, there are different types of GMT watches. How they’re set and how you read the time depends on your specific type of GMT watch. Here’s an overview of each of these options:

First Generation GMT watches

The earliest generation of GMT watches have their two-hour hands linked. This means one cannot be moved independently of the other. The regular-hour hand moves on a 12-hour scale, while the GMT hand moves on a 24-hour scale, meaning it is half as fast, but turning the crown moves both of them at the same time.

To set the time with this type of watch, you must set the hourly time to show the local time in your current location. Next, rotate the 24-hour bezel to align with the GMT hand. This way, the GMT hand points to the time in your desired time zone on the bezel.

Second-generation GMT Watches

A second-generation GMT watch is easier to understand since the hour hands can be moved independently. But even with this, there are two different options: the office GMT and the Caller GMT:

How to Set an Office GMT watch

With the office GMT watch (also known as a caller GMT), the foreign time zone is displayed with the GMT hand. For simple understanding, imagine you’re a businessman trying to call a branch of your business in a different time zone. You’ll want to set the time at the second location without disturbing your home location. This is how an office or caller GMT works.

In this case, you simply pull the crown to the second position (the position for setting the date if the watch has a date function). One direction changes the date while the other moves the GMT hand to your desired position. This does not affect the local time. However, these watches also have a third position for the crown, which allows both hands to move at the same time if you want.

Despite the name, an office GMT can still be used when traveling. It’s just a bit more complex to set. Say you’re flying from Paris to New York, which is a 6-hour time difference. If you land in Paris at 10 PM, the time in New York will be 4 pm. Since your watch is still set to Paris as your local time, the time displayed on the main hour hand will be 10 pm.

To adjust, you need to pull the crown to the third position, which allows you to move the local hand to 4 PM; then you can adjust the crown back to the second position and move the 24-hour hand to 10 PM, which is the time at home (Paris).

How to Set the Traveller GMT

The traveler GMT (the True-GMT) uses the GMT hand to display the home time while the other hour hand displays the local time. To set this type of GMT watch, the crown is pulled to the second position and turned in any direction to adjust the hour hands, but the GMT hand does not move. This way, reading the time with the hour hand shows the time in your current location while the GMT hand shows where you’re coming from. The third crown position allows you to adjust the two hands at the same time, and rotating the bezel makes it possible to track a third-time zone.

Conclusion

The GMT complication sounds complicated when you try to wrap your head around it. But you don’t have to be a watch nerd to figure out how it works. The most important thing is to know the type of GMT watch you own. This will determine how it will be set and how to read the time on it, whether you’re at home or traveling.

Understanding the GMT Function: What It Is and How It Works

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