What Is a GMT Function?
A GMT is a useful complication for travelers, international businesses, and those with family abroad. The complication is extremely popular and can be found across brands and models. Having a GMT is different than having a second timezone. Learn the differences and how to use your GMT function
Greenwich Mean Time
GMT stands for Greenwich Mean Time, which is the ‘zero hour’ standard according to the Royal Observatory in London. As you move around the world, you either add or subtract hours to get your local time. Hong Kong is GMT +8, Dubai is GMT +4, and Philadelphia is GMT -5.
In 1884, Greenwich was chosen as the point of zero degrees longitude, serving as international standard time and how local times were standardized. This was originally designed to help regulate train schedules, rather than using the sun positions as it will vary by location. The standardization helped avoid delays and also benefited naval navigation and eventually aviation.
In 1972, GMT was replaced by UTC, Coordinated Universal Time, which was based on atomic clocks and is not a timezone itself. Most watch manufacturers use GMT rather than UTC, although IWC has been known to use the designation.
How Does a GMT Watch Work?
The GMT function became popular in the 1950s when aviation began to take off. In fact, Pan Am approached Rolex to design a watch that will help pilots on long flights track the time from where they departed to the destination. From there, the Rolex GMT-Master ‘Pepsi‘ was born and it has now become one of the most iconic watches. The function adds a fourth hand to the dial, typically in a different color or shape for legibility.
The Scale
A GMT watch will feature a 24-hour scale, referencing the 24-hours in a day and completes just one rotation per day. A second timezone watch, often confused for a GMT, will feature a normal 12-hour scale and thus complete 2 rotations. The 24-hour scale also makes it easy to remember day versus night hours. The GMT-Master ‘Pepsi’ took this legibility to the next level and has a two-tone bezel highlighting the time of day; the red half is daytime while the blue is night.
Using the Bezel
Depending on your watch, the GMT hand may be anchored to the hour hand. As you adjust the local time, the GMT hand will move with it. For the second timezone, you would simply turn the bezel to align with the proper hour. There have been advancements in the movements and many brand’s product models with independent GMT hands. The bezel still rotates bi-directionally for easy setting of the time.
Not all GMT watches feature an hour-scale bezel. The Seamaster Aqua Terra GMT, for example, features the 24-hour scale printed between the indices on the dial.
Setting And Reading A GMT Watch
When setting a GMT with a static GMT hand you’ll want to start with the bezel aligned at 12 o’clock. The GMT hand will align with the current local time. To tell time in a different city, you would rotate the bezel either left or right, depending on the location relative to your timezone. If the second timezone is behind, you would rotate the bezel clockwise. If the timezone is ahead, you would rotate the bezel counter clockwise.
Using an Independent GMT Hand
A watch with an independent GMT hand, like the modern GMT-Master II, allows for quick movement of the hour hand, leaving the GMT hand static. To use this variation of the GMT function, set the GMT hand to local time, keeping the 24-hour scale in its original position with 12 o’clock aligned with 24. Then set the hour hand to the current time on the 12-hour scale. You can now use the bezel to align with the second timezone you are tracking. This is also how you would set a watch with the scale printed on the dial.
Using a GMT To Track A Third Timezone
Because many modern GMT functions feature an independent GMT hand, it is possible to track a third timezone by using the GMT hand and 24-hour bezel. To accomplish a third timezone, you will set the GMT hand to current GMT time, and the hour hand set to local time. The bezel can then be rotated to set a third timezone, using GMT as the reference for how many hours to add or subtract, rotate either clockwise or counterclockwise depending on location. The bezel would then align with the third timezone and GMT hand, your local time would be read by the hour hand. It is important to note that the second timezone would need to have the bezel rotated back for reading the time.
Can a GMT Be Used As a Compass?
Yes, it is true that a GMT watch can be used as a compass, highlighting either north or south depending on your hemisphere. By setting the GMT hand to local time, you would point it towards the sun. If you are in the northern hemisphere, the GMT hand will point north, if you are in the southern hemisphere, the GMT hand will point south.
Who Uses a GMT Watch and Why Own One
Outside of a dive watch, the GMT might be one of the most useful functions. This is especially true for those who travel often for work, or have family living in different timezones. It’s also a nice homage to the childhood dream of becoming a pilot, or even if you just like the way it looks.
Popular GMT Models
Because of its popularity, the GMT function is found throughout a number of brands and models. Some of the most popular GMT watches include:
- Rolex GMT-Master II
- Tudor Black Bay GMT
- Omega Seamster Aqua Terra GMT
- Rolex Sky-Dweller
- Grand Seiko Heritage Spring Drive GMT
- Breitling Chronomat
- Oris Aquis
Remember, not all dual time watches are GMT functions, there are also world times and second time zones. If the watch doesn’t have a 24-hour hand, you’ll want to make sure it has an AM/PM indicator. If this indicator is not available, it would purely be a dual time watch rather than a GMT. Discover the latest GMT watches at The 1916 Company or shop the entire collection of authorized new watches, pre-owned luxury watches, or Rolex Certified Pre-owned watches to find your perfect timepiece.