How to use a GMT

In horology, the GMT term refers to the ability to display the time across multiple time zones. Useful for anyone with business around the world, the GMT complication becomes an invaluable travel companion, coordinating and connecting people from every corner of the globe. For the purposes of this blog, we will reference the Tag Heuer Aquaracer GMT way201f.ba0927.

^{Tag Heuer Aquaracer GMT way201f.ba0927}

GMT watches are easily identifable as they traditionally feature an additional prominent hour hand in the centre and a bezel with two contrasting colours (the brighter signifying day and the darker, night). Printed on the bezel is a 24 hour display, allowing the wearer to distinguish between a.m. and p.m. in the second time zone.

^{The GMT hand and bi-colour bezel showing the 24 hour display.}

The hour and minute hands operate as usual, the minute hand fully rotating around the dial once an hour and the hour hand doing the same every 12 hours.

The hour hand for the second time zone moves slower, completing only 1 full rotation of the dial in 24 hours. In conjuction with the standard minute hand, it can be read to give the time in a selected second time zone. Assuming that both time zones are set to Greenwich Mean Time, the first and second hour hand will tell the same time. The first hand pointing to the hour on the 12 hour dial, whilst the second hand indicates the hour as it is printed on the 24 hour bezel. When travelling, either hands can be set to the ‘home’ or ‘local’ time.

^{When aligned to the same time zone, either hands can be used to read the time. Here it is 10:10 a.m.}

So how do you utilise this tool when travelling across the world? It is easily done via the following methods:

Bezel Rotation:

Make sure the main hour, minute and GMT hands are correctly set to your home time.
Make sure the bezel is set to the 12 o’clock position. The 12 is usually shown by a small triangle with a luminescent fill.
Simply rotate the bezel in the direction corresponding with the time difference in the local time zone, until the GMT hands points to the local time. The bezel rotates anti clockwise for timezones ahead of GMT and clockwise of timezones behind.
Read the hour as displayed on the bezel

It is as simple as that, providing you keep the watch wound and correctly set, you can just jump straight to step 3. Now let’s work through an example using the Aquaracer, assuming we are based at 1:30 p.m. GMT and are travelling to Athens, Greece in EET (GMT+2).

^{Both hands are set to GMT and the bezel is in the standard position. The time is 1:30 p.m.}

Make sure the standard hands are set to 1:30 p.m. The red GMT hand should be between the 6 and 7 o’clock markers.
Make sure the bezel is set to the 12 o’clock position with the circle and triangle at the top. In the standard position, the GMT hand should point between the 12 and 14.
Rotate the bezel two positions anti-clockwise so that the the GMT hands now points between the 14 and 16.
You should now read the time on the bezel, as pointed to by the GMT hand and the minute hand as 15:30, showing the local time in Athens.

^{The bezel has been moved 2 positions anti-clockwise, showing the local time in Athens as 15:30.}

Hour hand:

Brands will vary on how they operate this method of setting the GMT hand. Some will move the main hour hand independently with a twist of the crown, whilst others will use the crown to shift the GMT hand. The steps are as follows:

Make sure the main hour, minute and GMT hands are correctly set to your home time.
Make sure the bezel is set to the 12 o’clock position. The 12 is usually shown by a small triangle with a luminescent fill.
Unscrew and pull the crown out to the necessary position for changing the GMT hand.
Keep the bezel in the standard position. Rotate the crown, moving the GMT hand until it lines up with the hours on the bezel to to display the local time.

As before let’s work through an example using the way201f. ba0927. This time we will assume it is 3 p.m. GMT and we have just travelled to New York (GMT -5)

Make sure the standard hour, minute and GMT hand are set to 3 p.m.
Align the bezel back to the correct position with the triangle at the 12.
Unscrew the crown and adjust it to the second position.
Rotate the crown clockwise to move the GMT hand forward one position. Continue doing this until the GMT hands point to the 10 on the bezel.
Push the crown back into it’s position and screw it back down to ensure water resistance.

^{With the bezel remaining in the normal position, reading the GMT hand directly shows it is 10 a.m. in New York}

As with anything, it is important to read the manufacturer’s guidance on how their mechanisms work to ensure accurate time keeping and to prevent damage from occuring to the movement of your watch.

Most GMT’s will come with a date window. The date gears are usually linked to the main hour hand and will change when that goes past midnight. If you are choosing to use the main hour hand to indicate local time, be vigilant not to damage or offset the date feature.