A GMT bezel is a rotating bezel marked with 24 hours, paired with a 24-hour hand to show another time zone. Because the scale covers a full day, the wearer can tell whether it is day or night there. Turning the bezel also lets the wearer read a third time zone.
How does a GMT bezel work?
A GMT watch has an extra hand that goes round once a day, not twice like a normal hour hand. That hand points to the 24-hour scale on the bezel. To set the bezel for another zone, turn it by the number of hours that zone is offset from UTC. The 24-hour hand then gives the time in that zone against the bezel scale. The hand and the scale both cover a full day, so the reading also shows whether it is day or night in that place.
Why do some GMT bezels have two colours?
Some GMT bezels use two colours on the 24-hour scale: one for the daytime hours and one for the nighttime hours. The split shows at a glance whether the 24-hour hand points to day or night in the other time zone. On the original red-and-blue bezel of the Rolex GMT-Master, red stood for daytime and blue for nighttime. That first bezel was made of Bakelite.
Where it comes from
The name GMT comes from Greenwich Mean Time, the mean solar time at the Royal Observatory in Greenwich, London. Rolex released the GMT-Master in 1955. It was developed with the airline Pan Am for its pilots and navigators. The aim was to let pilots read local time and home time at once. The first GMT-Master fitted its Bakelite bezel to the case of the earlier Rolex Turn-O-Graph.
Not to be confused with
A watch with a fixed 24-hour bezel, such as the Rolex Explorer II, is really a dual-time watch rather than a true rotating-bezel GMT. A true GMT pairs its 24-hour hand with a bezel that rotates. The earlier Rolex Turn-O-Graph is also different. Its bezel had a 60-minute scale, not a 24-hour one. The GMT-Master’s bezel used a 24-hour scale instead.
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