====== GMT ======
{{wst>image_cl|Rolex|GMT-Master II|[[Rolex GMT-Master II]]\\ with the additional feature of a **GMT** function (green hand)}}

A **GMT** watch has a specialized [[dual timezone]] complication displays Greenwich Mean Time on a 24-hour scale in addition to local time.

===== GMT (Greenwich Mean Time) =====
On the Meridian Conference 1 October [[1884]] in Washington the [[local time]] of the zero meridian at the observatory of the (now incorporated) London suburb of Greenwich was confirmed as Universal Time (//Greenwich Mean Time//). Unlike the [[UTC]], which is based on atomic clocks, this is an astronomical, sun-oriented time convention, though the terms are often used interchangeably.

===== Watches with GMT function =====
{{wst>image_cr|TAG Heuer|Grand Carrera Calibre 8 RS}}

The abbreviation "GMT" means the possibility to display [[dual time zone|an additional time zone]] on a 24-hour scale on the watch dial. The terms "GMT", "dual timezone", and "[[worldtimer]]" are often used interchangeably by both watch buyers and manufacturers, but a GMT watch must show a 24-hour scale (or 12 hours plus [[day/night indicator]]). Although GMT and dual timezone watches often use the same movement, a GMT watch is intended to be set to GMT and left there, while the dual timezone watch is intended to have both zones adjusted more frequently. A worldtimer, by contrast, shows many timezones at once.

The classic GMT watch complication features an **extra hour hand** coaxially located at the center of the dial that uses a [[24 hour]] scale, as is the case with the original GMT watch, the [[Rolex GMT-Master]]. This format (two central hour hands, one of which has a 24 hour scale and the other which can be independently set on a 12 hour scale) is known as a "true GMT", though Rolex did not implement independent adjustment until 1980.

Other GMT watches use an independent 24-hour subdial on the face, as is the case with the [[TAG Heuer Grand Carrera Calibre 8 RS Grande-Date GMT]]. There are many other approaches, such as the [[Jaeger-LeCoultre Reverso Duoface]] which features a second dial on the back with a [[second timezone]] that can be used for GMT thanks to a 24 hour subdial.

The need for such models encreased in parallel to the greater volume of air traffic in the [[1940]]s and [[1950]]s, when more and more flights lead through several time zones. Note that some watches, such as the [[Rolex GMT-Master 6542]] of [[1954]], did not have a separately-adjustable hour hand. Instead, the [[bezel]] was rotated so the fixed 24-hour hand would reflect the correct time. It was not until [[1980]] that the [[Rolex GMT-Master 16750]] was released with an adjustable GMT hand. So although these earlier watches did display local time and GMT on a 24-hour scale, they were not mechanically GMT movements.

===== See also =====
  * {{tagpage>GMT_watches|GMT watches}}
  * [[Dual timezone]]
  * [[Worldtimer]]

===== External links =====
  * [[http://www.rmg.co.uk/explore/astronomy-and-time/astronomy-facts/history/the-prime-meridian-at-greenwich|Greenwich Meridian - Prime Meridian of the World]]
  * [[http://forums.timezone.com/index.php?t=tree&goto=6715286&rid=12189#msg_6715286|GMT Watches and the Nomos Zurich Weltzeit]]


{{tag>Abbreviations Terms}}