Jaeger-LeCoultre Reverso Duoface, back side
A dual timezone watch can display the time in two locations at once.
Dual timezone watches can display both local time and the time in another time zone. One popular use for this feature is known as GMT, in which a watch displays both local time and Greenwich Mean time on a 24 hour scale, a feature useful for pilots. GMT is a subset of dual timezone, however, that must have a 24 hour scale or day/night indicator for “home time”. Some dual timezone watches lack the 24 hour scale and simply have a second hour hand. Others have useful complications for travelers rather than pilots, including airport codes (as on the Nomos Tangomat GMT), a globe, and so on. For example, the Jaeger-LeCoultre Reverso Duoface has pushbutton adjustment for the rear dial, with the intention that a traveler will leave the front face at their “home” time and use the pushbutton to adjust the rear face as they travel.
Another related complication is the so-called worldtimer watch, which displays the time in many timezones at once. Many of these use the same dual timezone movement as a simple dual-time watch but include a disc in the face showing timezones around the world along with day and night indicators.
Some watches have a 24 hour hand or subdial that is locked to the main dial and are therefore not dual timezone or GMT watches.
The need for such models arose as air traffic became more popular in the 1940s and 1950s, when more and more flights lead through several time zones. Thus the Rolex GMT-Master, one of the classics of this watches genre, was developed in 1954 by Rolex as a result of specific requests from pilots to track GMT along with local time. The GMT-Master has a long hour hand that makes one revolution around the dial in 24 hours and points to a bezel marked with 24 hour increments as well as a short hour hand that can be moved to follow local time. 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”.
As travelers began purchasing GMT watches, they often set the “base” time to their home timezone rather than GMT. This led to the development of traveler-friendly complications like worldtimers. Since many such watches are not set to GMT, watches with extra 12-hour independent hour hands began appearing.