THE TRAINING CENTER

What Does GMT Mean?

The initials GMT stand for Greenwich Mean Time, the mean solar time at Greenwich, which was historically used as a reference point for determining time zones.

Today, UTC (Coordinated Universal Time) is primarily used as the scientific standard, but the term GMT has remained firmly established in watchmaking to describe a second-time-zone function.

The fundamental difference between a GMT watch and a conventional watch is the presence of an additional 24-hour hand.

The conventional hour hand completes one full rotation of the dial every 12 hours.

The GMT hand, by contrast, completes one full rotation every 24 hours.

For this reason, it is paired with a 24-hour scale. This scale can be located on the dial or, as is the case with many characteristic GMT watches, on the rotating bezel.

This allows the wearer to have:

• the local time from the conventional hour hand,
• a second time zone from the GMT hand,
• and, in certain designs, a third time zone using the rotating bezel.

How Does a GMT Mechanism Work?

The GMT function requires an additional gear system within the movement.

The GMT hand must operate at a different transmission ratio from the conventional hour hand so that it completes one rotation every 24 hours rather than every 12.

Put simply, the mechanism has to “know” that, for this particular hand, a full day corresponds to a single rotation.

This allows the GMT hand to indicate not only the hour but also whether that hour falls during the day or at night.

For example, if the hand points to 10 on the 24-hour scale, we know that it is 10:00 in the morning. If it points to 22, the time is 22:00, or 10:00 in the evening.

The 24-hour indication therefore eliminates one of the main ambiguities of a conventional 12-hour display.

Who Is a GMT Watch For?

The GMT function was created for people who need a practical way to keep track of more than one time zone.

A watch of this kind can be particularly useful for:

• Pilots and aviation professionals, who frequently travel between different time zones.
• Frequent travelers, who want to know both the time at their current location and the time at home.
• Sailors, who may need to refer to different time zones during a journey.
• Professionals with international responsibilities, who communicate with colleagues or clients in other countries on a daily basis.
• Watch collectors, for whom the functionality and mechanical complexity of a GMT are themselves important characteristics.

For someone who travels once or twice a year, the function may not be essential. For someone who frequently moves between different continents, however, it can become particularly practical.

How Do We Use a GMT Watch When Traveling?

Let us assume that someone is traveling from Athens to New York.

Before the trip, they may have set the GMT hand to display Athens time, using it as their reference time.

Upon arriving in New York, on a GMT watch with an independently adjustable local hour hand, they can move the conventional hour hand to the local time in New York without changing the GMT indication.

This means that, at a glance, they can see:

• Conventional hour hand → New York
• GMT hand → Athens

This is the fundamental advantage of the function.

There is no need to constantly remember or calculate the time difference between the two cities.

How Do We Set the Rotating Bezel to Read a Third Time Zone?

This is one of the most interesting features found on certain GMT watches.

First, however, an important clarification is necessary:

Not every GMT watch can display three time zones.

To do so, the particular design must feature a suitable rotating 24-hour bezel that can work in conjunction with the GMT hand.

A characteristic example is the Rolex GMT-Master II. Rolex specifically describes the use of its rotating bezel to indicate an alternative time zone.

The procedure is as follows.

1. Set the Bezel to Zero
First, position the bezel in its neutral position.
This means that the triangle on the bezel should be positioned directly opposite the 12 o’clock marker on the dial.
In this position, the GMT hand is read normally against the 24-hour scale on the bezel and indicates thereference time.

2. Calculate the Time Difference
Next, calculate the number of hours between the time zone you want to display and the reference time.
This is where particular attention is required:
If the third time zone is east of the reference time and ahead of it, rotate the bezel counterclockwise.
If the third time zone is west of the reference time and behind it, rotate the bezel clockwise.
In both cases, move the bezel by the same number of clicks as the number of hours in the time difference. On the GMT-Master II, the bezel has 24 hourly positions.

3. Read the GMT Hand Against the New Bezel Position
The important point is that you do not move the GMT hand.
You move only the bezel.
The GMT hand continues to move normally, completing one rotation every 24 hours. However, because it is now positioned opposite different markings on the bezel, it provides the time in the new time zone.

In other words:

The hand remains constant in its movement — we move the scale underneath it.

That is the “trick” behind the function.

A Specific Example

Let us assume that our reference time is Geneva and that we want to know the time in Tokyo at the same time.

Tokyo is 8 hours ahead of Geneva.

Initially:

1. Position the triangle on the bezel at 12 o’clock.
2. The GMT hand indicates the time in Geneva.
3. Because Tokyo is 8 hours ahead, rotate the bezel 8 clicks counterclockwise.
4. Now read the same GMT hand against the new position of the 24-hour scale.

The resulting indication is the time in Tokyo.

This is precisely the example provided by Rolex in its GMT-Master II instructions: when Geneva is used as the reference time and the wearer wants to display Tokyo time, the bezel is moved 8 positions counterclockwise.

Similarly, if we want to display a time zone that is, for example, 5 hours behind the reference time, we rotate the bezel 5 clicks clockwise.

In One Sentence:

Ahead → counterclockwise.
Behind → clockwise.
One hour of difference → one click.

This is perhaps the simplest “formula” to remember how the function works.

And How Do We Read Three Times Simultaneously?

With a GMT-Master II, we can have two basic indications simultaneously when the bezel is in its neutral position:

Conventional hour hand → local time

GMT hand + 24-hour bezel in the zero position → reference time

However, if we rotate the bezel to display a third time zone:

Conventional hour hand → local time

GMT hand + shifted bezel → alternative third time zone

In the latter case, the reference time is no longer displayed by the bezel because its scale has been moved. This is an important point that can often cause confusion.

Therefore, the ability to display a “third time zone” does not mean that we have three independent hands showing three different times. Instead, we have two time-indicating hands and one movable 24-hour scale, which allows the GMT hand to be used to display a different time zone.

Why Does the Bezel Have 24 Hours?

The answer lies in the GMT hand itself.

Since the hand completes one rotation every 24 hours, it must be read against a corresponding 24-hour scale.

The rotating bezel essentially acts as a movable reference point.

By moving it, we change the relationship between the GMT hand and the 24-hour scale.

On the Rolex GMT-Master II, the bezel rotates bidirectionally and “locks” into 24 different hourly positions. Its design allows the wearer to move it precisely one hour at a time.

GMT and Daylight Saving Time

There is, of course, another factor that must be taken into account when using a GMT watch in real-world situations: daylight saving time.

The difference between two countries is not necessarily constant throughout the year, since some countries observe changes between daylight saving time and standard time while others do not.

Therefore, when using a GMT watch for travel or for communicating with another country, we need to know the actual time difference on the specific date in question.

The watch mechanism itself, naturally, does not “know” which country observes daylight saving time. Correctly adjusting the watch remains the responsibility of the wearer.

A Mechanical Tool on the Wrist

The GMT function is a perfect example of the philosophy behind many important developments in watchmaking.

It is a function that was created to address a real need: the need to know the time in more than one time zone.

Today, a mobile phone can provide this information within seconds. Nevertheless, a mechanical GMT watch continues to have a particular appeal, precisely because the same information is generated by a system of gears, shafts, hands, and mechanical components operating without electronic circuits.

And in the case of a GMT watch with a rotating 24-hour bezel, the function becomes even more fascinating.

With just one additional hand and an intelligently designed bezel, the watch can give us access to more than one time zone, transforming a complex mechanical system into a simple and practical everyday function.

Perhaps this is also the best way to understand what high watchmaking really means: not simply more components, but intelligent engineering that transforms a complex function into something simple and intuitive for the person wearing the watch.