How GMT Watches Work Part 1 – Basics

Watches with GMT display, i.e. a second time zone, have been enjoying increasing popularity for years. We want to take a look behind the scenes here and get to know different variants of the realisation of a GMT display.

To start with, a very brief historical outline on the subject of GMT:
In 1954, Pan American Airways commissioned the Rolex company to develop a timepiece that would display a second zone time. On transatlantic flights, pilots pass through several time zones. At that time, both the world time “Greenwich Mean Time” (GMT) and the local time at the destination had to be available to the pilots at the same time. The result was the Rolex model 6542, the GMT-Master.

In 1972, UTC (Universal Time Coordinated) was introduced as the world time, making GMT actually superfluous. Today, GMT is mainly used as a term for a second time zone, since hardly anyone really wants UTC displayed, but the time of an arbitrary time zone, such as the one at home.

In principle, there are three main variants of GMT displays:

1. Pseudo-GMT

The normal hour hand and the 24 h hand always move synchronously, so the two hands cannot be set independently of each other. One turns once in a circle in 12 hours, the other in 24, so both always show the same time on the dial, one in 12-hour format, the other in 24-hour format.
In this case, a rotating bezel with a 24 h display is needed to turn it to the 24 h hand so that the desired time zone is displayed on the bezel.
The watch shown here, a Nautec No Limit, works like this:

This is the simplest and cheapest way to realise a 24 h display. If we are honest, the movement itself cannot actually display a second time zone. Only the rotating bezel makes this possible, which is why I call this variant pseudo-GMT. Incidentally, the original Rolex GMT-Master also worked according to this principle.

2. “I do not travel” or “caller” GMT

The normal hour hand and the 24 h hand can be set separately. The minute hand, the second hand and the date switching are connected to the normal hour hand. The 24-hour hand can usually only be set in units of full hours relative to the hour hand.
The 24-hour hand can be adjusted on an extra crown position, while the normal movement simply continues to run. This is how the ETA 2893 works, for example.

If, for example after a flight, you wanted to set the local time at your destination, i.e. x hours ahead or back, you would turn the normal hour hand so that the date display also switches correctly. However, because of the stop-second, which is usually present, the watch stops during the setting process. It must therefore be completely reset at the destination, i.e. hour, minute, second. This is practicable, but somewhat impractical.

However, this variant is practical if you want to keep an eye on the time zone in a place other than your own location via the 24 h hand, for example that of a work colleague abroad whom you want to call. That is why I have chosen the designation “I am not travelling” or “caller” GMT.
We will look at this Orient Classic Automatic Power Reserve GMT later, which works according to the principle I just described:

3. “I travel” or “real” GMT

The normal hour hand and the 24-hour hand can also be set separately here. The minute hand and the second hand are connected to the 24-hour hand, whereas the date switching is connected to the normal hour hand. The hour hand can usually only be set in units of full hours relative to the 24 h hand.

With the true GMT movements, in contrast to variant 2, you can now adjust the normal hour hand and thus also the date, while the minute, second and 24-hour hands continue to move. The date even changes backwards here if you turn the hour hand back over the date line.

This is practical when you are travelling yourself and set the time to your destination after the flight without having to bother with the stop-second and the associated setting of the second and minute hands. This also explains the chosen term “I travel” GMT.
Unlike the old Rolex GMT Master, the newer GMT Master II has a true GMT display in the sense described here. The Omega Seamaster Professional GMT, the Grand Seiko GMT models and this IWC Ingenieur Dual Time also belong to this variant:

At the end of the day, however, all three variants fulfil their purpose of displaying a second time zone!

However, variants 2 and 3 of a GMT display have a small disadvantage: they cannot display time zones with differences that do not correspond to full hours between the normal hour hand and the 24-hour hand. There are, however, time zones with 30 or even 15 minutes difference to the full hour, for example in India and Nepal. This is not a problem for version 1, as the external bezel can be rotated as desired.

So here we would need a watch where both times can be set individually in hour and minute. There is of course something like that, such as the Rainer Brand Panama Dual Time:

So, that was the first part. In the second part, we will take a closer look at the technology behind the GMT display. See you soon!