Multiple time zones and GMT

A dual-time function – also referred to as GMT, short for Greenwich Mean Time -, allows travellers to set their watch to time at their destination (local time or travel time) while still being able to easily consult time in their home city. A multiple time zone watch displays the time in three or even four different points on the globe. This is a useful complication for anyone who travels internationally and should not be confused with world time, which gives a permanent indication of local time in 24 world cities in relation to a fixed reference point (home time).

Local time is usually shown by the watch's central hour and minute hands, which obliges the user to reset the time whenever he or she moves to a different time zone. This can be facilitated (and the risk of damaging the movement reduced) by a mechanism, operated by the crown or by +/- pushers, that moves the hour hand (and only the hour hand) forwards or backwards in one-hour increments.

The majority of dual-time watches only show the hours of home time, based on the principle that almost all time zones are offset by one hour. A few makers also incorporate an additional display for minutes to take into account the handful of countries which have a 15-, 30- or 45-minute offset. This also means home time can be set to within one minute.

The most popular means of displaying home time are by an additional hour hand on the main dial (in a different colour to distinguish it from the hour hand for local time), on a subdial showing the hours and possibly minutes, or on an additional hour display in an aperture. Some watches use a 24-hour scale on the bezel or a rotating flange. Once local time is set, the wearer turns the bezel or flange to align the corresponding numeral for home time with the hour hand. This solution can be combined with one of the other methods to add a third time zone. A rare few watches enable the wearer to select cities: the name of the chosen city appears in an aperture while the home time hand (or hands) automatically set to this time.

As well as knowing what time it is while away from home, the wearer of the watch also needs to know if it is daytime or night. If home time is shown over 24 hours, the problem is resolved. If, on the other hand, the home time display is over 12 hours, it should be combined with a day/night indication (sometimes called an AM/PM indication) or an additional 24-hour counter.

Some multiple time zone watches have several separate dials, each with its own movement. This solution isn't considered to be a complication as each movement has just a basic construction. In contrast, a watch that shows a second time zone over 12 hours will have a GMT wheel connected to the hour wheel. The two rotate at the same speed. A clutch system on the GMT wheel means local time can be set without modifying the other displays. When time is shown over 24 hours, a reduction pinion rotates the GMT wheel in 24 hours as opposed to 12 hours for the hour wheel.

Until the late nineteenth century, there were thousands of different local times. Isolated one from the other, every town, every region, woke and went to bed to its own time, based on the position of the Sun in the sky (solar time or true time). A traveller making a journey of just a few miles would have to reset their watch to the new local solar time.

Rail travel - which began in the United States in 1830 with the first section operated by the Baltimore and Ohio Railroad and in Europe in 1838 with the London and Birmingham Railway - together with Samuel Morse's (1791-1872) invention of the telegraph led to more, as well as faster, communication between not just towns but countries too. The inconvenience of multiple time zones became all the more obvious, especially in countries with a large east-west expanse such as the United States whose many private railroads each ran to its own time. And so it was that in 1883 railway companies in the US and Canada agreed to divide their territory into five equal zones, thus introducing the entirely new concept of "time zone". The following year, delegates to the International Meridian Conference in Washington succeeded in reaching an agreement whereby the Earth was divided into 24 one-hour zones, calculated from a prime meridian that ran through the London borough of Greenwich.

Greenwich Mean Time (GMT) was thus adopted as the base for civil time. Calculated from true solar time in Greenwich, it would play an important role in conquering the skies. Indeed, rail travel shortened distances in the nineteenth century but this would be nothing compared with the airplanes that made long-haul flight a reality in the twentieth century. The ability to connect two cities thousands of miles apart in just a few hours demanded a single reference time. This would be GMT, used by pilots and air traffic controllers for calculating flight plans.

Launched in 1940, Universal's Aéro-Compax was the first chronograph to be made for pilots, specifically Brazilian pilots. In addition to 30-minute and 12-hour counters, it features a subdial at 12 o'clock with hour and minute hands that could be set, by means of a crown at 9 o'clock, to a reference time (such as the start time of a mission). The hands on this subdial weren't connected to the movement, hence didn't move.

In 1954 Rolex introduced the GMT-Master which prefigured all future GMT watches. Developed in partnership with Pan American Airways (better known as Pan Am), it has a second central hour hand that makes one rotation in 24 hours. An arrow tip points to a 24-hour scale on the bezel. The other hour hand can be separately adjusted, thanks to a clutch.

Subsequently, a multitude of brands would roll out their own dual-time or multiple time zone watches. Heuer presented the Twin-Time in 1955 along the same lines as the GMT-Master but with a second hour hand that rotated over 12 hours. Omega launched the Flightmaster, the first chronograph with a GMT hand, in 1969. Rolex reissued its historic dual-time in 1982 as the GMT-Master II. And in 1994 Ulysse Nardin became the first to use two pushers to move the hour hand forwards or backwards in one-hour jumps on the GMT +/-.

Since 1972 GMT has been replaced by UTC (Coordinated Universal Time) which is based on international atomic time. Even so, watchmakers continue to refer to their dual-time watches as GMT.