The GMT complication was created for Pan Am pilots but has gone on to become one of the more popular and useful functions in modern watch making. It is essentially an independent hour hand which tracks a twenty four hour time period rather than a normal twelve. This means you can at a glance track two time zones. The idea was if you set the twenty four hour hand to GMT (also known as Zulu Time or Z Time) then you could track up to three time zones. Your local time which will be displayed by the hour, minute and second hand, GMT which will be tracked by the twenty four hour hand and a third time zone with the timing bezel. All you have to do is rotate the bezel how many hours ahead or behind of GMT the time zone you are looking for is.
For example, say you are in Brazil which is three hours behind GMT and as such your hour hand is saying it is midday and your GMT hand is saying it is 3pm. If you then wanted to know what time it is in Greece (which is three hours ahead of GMT), all you would have to do is rotate the bezel backwards three hours and the GMT will now be pointing at 6pm which is the time in Greece. As you can imagine this was very useful for pilots and frequent flyers as it let them track home time and local time.
As technology improves the need for a GMT has softened but it still remains a very easy and quick way to know two time zones at once. Aside from the more obvious use around trips abroad and general international travel you also have a business application. Say you work for a multinational company or are dealing with a company based in a different country, a GMT affords you the ability to know what time both camps are on.
Despite GMTs history starting with Pan Am they have since grown and moved on to become their own thing. Starting in the fifties with the original Rolex GMT-Master which used Bakelite for the bicoloured bezel (two years later they switched to aluminium as the Bakelite kept cracking) they would see the GMT grow into a very popular product line. Years later the movement was upgraded and the GMT-Master was replace with the GMT-Master II which itself became a smash hit. An interesting reference of the GMT is 16760 which had two GMT hands which let you track another time zone. This watch went on to become incredibly popular, particularly among members of the air force, NASA and pilots.
Rolex has the closest association the GMT complication with their GMT Master being the most widely known model, but many other brands have got a version out. At time of writing Seiko has just released a new SKX with a GMT movement in it, as have multiple microbrands. Isotope specialises in GMTs and have an inner rotating bezel rather than a fourth hand to display the information. Higher end brands have also released multiple GMTs such as Montblanc, Omega, Breitling and Tudor.
GMT movements broadly fall into two categories, “True GMTs” and “Collar GMTs”. Despite how it sounds one isn’t inherently superior over the other they are different. Higher end watches do tend to be “True GMTs” and less expensive watches “Collar GMTs” but it isn’t a major difference. In a “True GMT” when you pull the crown out to first position you can move the hour hand independent of the GMT hand, this lets you change your local time zone and keep the GMT hand on GMT time. A “Collar GMT” does the opposite. The first position lets you change the GMT hand and keep the hour hand in place. A “True GMT” is better for travellers who frequently leave and enter new time zones as the GMT hand stays accurate. If however you stay in the same time zone and need to different time zones than the collar is better.