The concept of the leap second has only been existence since 1972, when Universal Coordinated Time was created. 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. In order to maintain this maximum tolerance of 0.9 seconds, mankind therefore invented “leap seconds”. Over time, the gap can sometimes grow, because of the Earth’s irregular trajectory, earthquakes, etc. Nevertheless, theory sometimes encounters practical limitations. That is why it was decided once and for all that GMT and UTC must never differ by more than 0.9 seconds. The trick is therefore to ensure that they remain as close to each other as possible. “UTC, 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. It’s a reference for time zones – a sort of zone zero,” explains Joël Petetin from the Chronometry division of Besançon Observatory. It has no direct relation to summer or winter time. To keep things very simple, GMT or Greenwich Mean Time represents the reference time zone. 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? Nowadays, there are more sophisticated movements and they can be generally divided into two types.A few days ago, WorldTempus looked at the difference between GMT, UTC, Worldtimer, Dual Time, Universal Time and other variations on the theme. To set a second time zone, it was thus necessary to turn the bezel. The earliest GMT watches were simple beasts: the 24-hour hand was anchored to the hour hand so that as you adjusted your local time, the GMT hand moved in tandem, thus tracking the local time as well, but on a 24-hour scale. Wearing it branded one as a traveler, someone who crossed time meridian lines and explored exotic corners of the world. But the Rolex, and other watches that followed suit, trickled down to coach class too, as jetset travelers saw the obvious advantage of a two-time zone watch, as well as the image it projected. The GMT-Master became the de facto pilot’s watch, and was seen on the wrists of fighter jocks and astronauts alike. If the Breitling Navitimer chronograph was the definitive pilot’s watch of the 1950s, its slide rule bezel and busy dial made it a quaint relic by the time the Jet Age hit its stride in the ‘60s. So those Pan Am pilots would always have their 24-hand set to GMT, no matter what their local time was. But if the bezel allowed for the tracking of any second time zone, why was the watch called the “GMT” Master? As part of their navigation and communication protocols, pilots always operate on GMT (or UTC) time, to eliminate any confusion. And because the bezel turns, any hour can be set to correspond to the GMT hand, thus instantly tracking a second time zone utterly genius, incredibly simple. The so-called “GMT hand” points to the 24-hour time scale on that bezel (blue for nighttime hours, red for daylight). The iconic red-and-blue “Pepsi” bezel that is so associated with the early Rolex GMT-Master was Rolex’s first solution to the two-timing conundrum. In other words, the 24-hour hand circles the dial once a day instead of twice, its tip pointing to the corresponding hour, leaving no confusion as to whether it’s a.m.
Because the movement's going train (primary series of gears) in a conventional watch is made to spin the hour hand around the dial every 12 hours, all that is required to become a GMT tracker is simply a second hour hand geared to run half as fast and an additional 24-hour time scale. The GMT watch is brilliant, not because of its complexity but because of its simple ingenuity.
But the name “GMT” stuck and still somehow conjures up the dawn of the Jet Age, when knowing where you were in relation to a stripe on the ground in Old Blighty was first made important, just as the famous red-and-blue bezel of that first GMT-Master watch on the wrist of a Pan Am captain does. By 1955, the atomic clock had been invented, and in 1960 the US and UK synched their atomic radio time signals into what would become Universal Time Coordinated, or UTC, which effectively replaced that meridian line that passes through the Royal Observatory in Greenwich.