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, a feature useful for pilots. GMT is a subset of dual timezone, however, and some dual timezone watches have useful complications for travelers rather than pilots. 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.
The 24 hour subdial on some watches can function as a dual timezone display while others are locked to the main dial.
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. 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.
Watches with a second timezone display often bear the name GMT or UTC, since this is the original use for this complication. Others use some variant of worldtimer as their name. Although all GMT watches are dual timezone, they are not correctly classified as worldtimers.