There is much confusion regarding watches with a second-time zone function, commonly referred to as "GMT." Let's clarify them in this article.
What Does GMT Mean?
GMT stands for Greenwich Mean Time, which corresponds to the mean solar time at the Greenwich meridian, the prime meridian for longitudes, passing through the Royal Observatory in Greenwich, near London, United Kingdom.
Greenwich Mean Time served as the worldwide time reference for most of the 20th century before being replaced by Coordinated Universal Time (UTC) in 1972.
Commonly, GMT is used interchangeably with the UTC+0 time zone, also known as Western European Time. However, these two time measurements, while close, do not coincide because GMT is based on the Earth's rotation, while UTC is based on international atomic time.
Greenwich Mean Time corresponds to winter time in the UK:
- At 3:00 PM GMT in winter, it is 3:00 PM in London.
- At 3:00 PM in summer, it is 4:00 PM in London.
With that clarified, let's move on to the different forms of watches with time zone functions and their use.
When Did Watches with Multiple Time Zones First Appear?
The invention of time zones on a timepiece is relatively recent, dating back less than 140 years. The reason for this horological development takes us to a time when long-distance travel became fast and accessible to all. We are in the era of the first transnational and transcontinental railway lines stretching over hundreds and thousands of kilometers. It was necessary to agree on fixed international schedules, whereas previously, each region or country could set its local time based on solar time on a planet with 24 different time zones.
At the origin of the famous GMT is an engineer named Sir Sandford Fleming, a Canadian of Scottish origin. His proposal for the standardization of international time was adopted at the International Meridian Conference in Washington in 1884. Twenty-five participating nations adhered to his principle and set the longitude of Greenwich, England, as the zero meridian.
The First Dual Time Zone Watches
The inventor of the first dual-time zone watch is none other than... Since watch manufacturers often claim the invention of various complications, I prefer to present you with a range of watches that offer functions to display multiple times in different countries.
Illustration of a Longines dual-time zone watch
This Longines watch from 1908, shown above, is claimed to be the first watch with a second-time zone. Indeed, no other manufacturer has been able to document a timepiece that simultaneously displays the time in two different locations around the globe.
A Rolex from 1943. This watch displays the time in no less than 31 world capitals and has a 24-hour day/night disc.
The first Rolex GMT-Master reference 6542 from 1954 with GMT hand and rotating 24-hour disc
Using a "Simplified" GMT Function
The simplest and most affordable function among inventions for reading the time in another country belongs to Rolex with its GMT-Master watch. Why the simplest? Mechanically speaking, all that's needed is to add a second rotating hand and a 24-hour rotating disc to easily indicate the time in another country. This cost-effective manufacturing step allowed the brand to become the absolute leader in GMT watch sales, a position it still holds today.
And it's the operation of this system that we'll focus on in this article, even though it has been improved over the years by the Geneva-based manufacturer.
How a Rolex GMT Master Works
The operation of the Rolex GMT-Master is quite simple. Let's take the example of a reference 1675, shown in the image...
For operation, here's an example. Let's say it's 4:10 PM in Paris, and your flight is taking off for New York. Set your watch to the local time in New York, which is 6 hours behind. It's now 10:10 AM in New York. Turn the disc until the local time in Paris is aligned with the red GMT hand, which is at 16 for 4:00 PM. Now you have the time in Paris and the time in New York. The hour hand will advance normally on a 12-hour scale, and the GMT hand on a 24-hour scale, which is half as fast. The two different times will be synchronized.
You have a 7-hour flight and arrive in New York at 5:10 PM local time (which is 11:10 PM in Paris).
