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What is GMT?

GMT is an acronym meaning Greenwich Mean Time.  GMT is the average solar time at The Royal Greenwich Observatory, which is on longitude marker 0 degrees (aka: Prime Meridian).  In 1960, GMT was officially supplanted by another standard, Coordinated Universal Time (UTC for short). UTC marks a major difference from GMT in the way the time itself is calculated.  It uses the major advancement of atomic frequency time scale clocks to replace the less accurate astronomical observations used in Greenwich.   Since UTC was adapted after the development of the so-called GMT watch, the acronym didn’t really catch on in the watch world. So, for the rest of this post, I will use the name GMT.
What is a GMT / UTC Watch?
A GMT/UTC watch is a watch that features a complication that allows for the wearer to keep track of a second time zone.  This could be GMT/UTC, but in normal use it tends to be the time zone of another office or the home-time of a traveller.  Since none of the GMT watches on the market today actually require tracking GMT, we believe the more accurate description for GMT / UTC  watches would be a dual time zone watch. Whether it's to monitor GMT itself or to track two different time zones is the user's preference.

Who made the first GMT Watch?
Well, it is strongly believed that the Rolex GMT-Master was the first watch ever released that could mark 2 time zones at once. Rolex released this watch in 1954 and was assisted by Pan American Airways (PanAm) pilots to determine what features they found essential. The feature deemed most important was the ability to track the time at origin and destination. Rolex achieved this using an additional hand and bezel divided into 24 hours. The bezel could be aligned with the additional hand relative to the
difference in time between an origin and a destination. 
The other watch brand that can lay claim to making the first dual time watch is, surprisingly, Glycine.  This small manufacturer began producing its Airman watch in 1953 but was eclipsed in the marketplace by the larger watchmaker. Glycine’s solution was, in my opinion, slightly more elegant. It featured a 24-hour dial, with the hour hand making one rotation per day. This was paired with a 24-hour engraved bezel, secured with a gate and thumbscrew between the crown and the lower lug. Functionally, it was
quite like the GMT-Master, but did not require a 4th hand on the dial.