A few days ago, WorldTempus looked at the difference between GMT, UTC, Worldtimer, Dual Time, Universal Time and other variations on the theme. In everyday life, however, the issue that raises the most questions is – what is the difference between GMT and UTC? Are they exactly the same thing?
What the experts say
To keep things very simple, GMT or Greenwich Mean Time represents the reference time zone. “It’s an average solar hour. It has no direct relation to summer or winter time. It’s a reference for time zones – a sort of zone zero,” explains Joël Petetin from the Chronometry division of Besançon Observatory. “UTC [Universal Coordinated Time], on the other hand, is based on coordinated times that can be considered invariable, because they themselves are based on International Atomic Time (TAI), which is calculated by a network of more than 350 atomic clocks around the world, from which an average is produced.” In concrete terms, this means that we have two reference times. The trick is therefore to ensure that they remain as close to each other as possible. That is why it was decided once and for all that GMT and UTC must never differ by more than 0.9 seconds.
Nevertheless, theory sometimes encounters practical limitations. Over time, the gap can sometimes grow, because of the Earth’s irregular trajectory, earthquakes, etc. In order to maintain this maximum tolerance of 0.9 seconds, mankind therefore invented “leap seconds”. They are way of artificially compensating for the slowing trajectory of the Earth and, incidentally, of maintaining this 0.9 second difference between GMT and UTC. The concept of the leap second has only been existence since 1972, when Universal Coordinated Time was created. In the intervening 45 years, 37 leap seconds have been inserted, the last one on 31 December 2016.
In terms of mechanical watchmaking, these differences are utterly irrelevant. It is possible to conceive of a mechanical timepiece that would be technically capable of adding a leap second every X years, “But that would be completely pointless,” interjects Carole Kasapi, Head of Manufacture Developments for Cartier. “It would be the equivalent of, for example, adding one second every three years to a mechanical watch, which might vary by five or six seconds… each day!”
In reality, GMT and UTC watches offer exactly the same functionality and precision. It is merely a semantic variation of the same mechanical function, which is the ability to display a second hour. Eberhard & Co., for example, has opted for the GMT label for its latest Scafograf, while Montblanc chose UTC for its most recent Timewalker. Laurent Ferrier has found a clever way of sidestepping the entire debate: its watch with two time zones is called the Galet… Traveller!