Mechanical principles › Complications › Time zones

THE DUAL TIME ZONE (GMT)

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1. Definition and purpose of the complication

The dual time zone (GMT) watch simultaneously displays the time of two distinct locations. One of the two indications represents local time. The other represents a reference time chosen by the user.

This complication meets a need born with modern transportation. Travelers, pilots, sailors, and international professionals regularly check two different times. The dual time zone (GMT) watch spares them any mental calculation.

The complication does not incorporate geographic data. It does not identify time zones by name. It simply displays two indications offset by a whole number of hours, set manually.

It differs fundamentally from the world time / universal time watch. The latter displays the time of all twenty-four time zones. World time / universal time watches will be covered in a separate documentseparate document.

2. Time zones: astronomical basis

The earth makes a complete rotation on itself in twenty-four hours. This movement produces the alternation of day and night. It is at the origin of the notion of Time zone.

The earth’s surface is divided into twenty-four theoretical time zones. Each zone corresponds to fifteen degrees of Longitude. Legal time differs by a full hour between two adjacent zones.

This division is an international convention established in 1884. The Washington Conference selected the Greenwich meridian as the reference. This meridian defines the planet’s zero Longitude.

In practice, time zone boundaries do not follow meridians. Political and geographical divisions deviate them considerably. A single country can span several time zones with one legal time.

Some states apply offsets of thirty or fifteen minutes. These quarter zones or half zones exist in India, Iran, and central Australia. They constitute special cases in the management of Time zones.

3. GMT and UTC references

Coordinated Universal Time (UTC) is the world’s time reference. Since 1972 it has replaced Greenwich Mean Time, known as GMT. Both abbreviations are used interchangeably in horological literature, although the term GMT should, since 1972, be considered incorrect.

UTC is maintained by atomic clocks at the International Bureau of Weights and Measures. Its accuracy is on the order of a nanosecond. It serves as the reference for navigation, aviation, and telecommunications.

Each time zone is defined by its offset from UTC. Paris is at UTC+1 in winter and UTC+2 in summer. New York is at UTC−5 in winter and UTC−4 in summer.

4. Display variants for the second time zone

4.1 The coaxial 24-hour hand

The coaxial second time zone’s hour hand rotates in 24 hours on the main dial. Its indication is immediate and readable at a glance. It is the most widespread solution in watchmaking production.

It requires a 24-hour graduation visible on the dial’s flange or the Bezel of the Case. A darker area of this graduation is generally present to identify nighttime hours (between 6 p.m. and 6 a.m.).

The Train (wheel train) driving the second time zone Hand is derived from the hours Train (wheel train). Its reduction ratio is 1:2 relative to the hour Hand. This additional train is simple, compact, and its energy consumption is negligible.

The shape and/or color of the second time zone Hand distinguishes it from the other hands.

The reading is immediate. The second time zone Hand points to the time of the second Time zone on the 24-hour graduation. Sometimes, the relevant area of the Dial is indicated by a nuanced coloring depending on whether it is day or night in that Time zone.

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4.2 The 12-hour Display with day/night Indicator

Sometimes the display of the second time zone is done on a twelve-hour cycle. At the centre as in the previous example or on a small autonomous subsidiary dial that displays the second time zone with its own hands. Its legibility is lower compared to a central display. This type of display generally rotates over twelve hours. As it does not distinguish between daytime and nighttime hours, it is generally complemented by a day/night indicator with an aperture, thus avoiding any unfortunate confusion.

Second time zone on 12 h off-centre (6 h) with day/night indicator with aperture (6 h)

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4.3 The time aperture

Some calibers display the second time zone in a digital window. A disc bearing the numbers from 0 to 23 advances in one-hour jumps or in a trailing manner. The indication is clear and unambiguous to read.

This solution is compact and does not overload the dial. It is suited to movements with limited available space or displaying a large amount of information.

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5. Correcting the second time zone via the crown

Correcting the second time zone must move the second time zone hand in one-hour steps. It must be possible without stopping the watch. The minutes must remain unchanged during each correction.

Correction via the crown is the simplest solution. The crown is pulled to an intermediate position. Turning it moves the second time zone hand forward or backward hour by hour.

The design of a multi-position Crown is a precise mechanical challenge. Each Crown position must control a distinct function and requires a construction specific to that Complication.

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6. Correction by pusher(s)

Correction by pusher is faster than correction by Crown. A Pusher in the case middle advances the hour Hand of the second Time zone by one hour with each press. A second Pusher allows it to go back.

The internal mechanism relies on a Click and Star (wheel) device. Each press moves the Star (wheel) forward or backward by one step. The Star (wheel) moves the hour Hand of the second Time zone by exactly one hour.

The ability to correct in both directions is valuable. It facilitates east-west travel as much as west-east travel. It avoids having to go through the entire 24-hour cycle to move back one hour.

A neck chain (sautoir) holds the star (wheel) in place between two successive presses. This spring ensures that the hour hand of the second time zone stops exactly on each hour.

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7. Seasonal time change

The seasonal time change requires a manual adjustment of the time on two time zones. This change is one hour, in one direction or the other, depending on each country’s calendar.

With a few rare exceptions, a mechanical watch does not automatically incorporate this change. The user must correct one of the two displayed time zones. The jumping hour mechanism makes this correction quick and precise.

The situation becomes more complicated when the two time zones correspond to regions with different calendars. The day of the time change varies from country to country. Some countries, such as Japan, do not apply any seasonal change.

8. Initial setting and practical use

The initial setting of a dual time zone watch is done in two steps. The first consists of setting the main hands to local time. The second consists of positioning the second time zone to the desired reference time.

The offset between the two time zones is determined in advance by the user. It corresponds to the difference in hours between the two chosen locations. This offset remains fixed until the user changes it.

When traveling eastward, the time moves forward. The user must decide whether to correct the main Hands or the second Time zone. This decision depends on the chosen usage convention and the construction of the mechanism.

Correcting the main Hands without disturbing the minute Regulation / Adjustment is an important requirement. Most Movements allow this correction hour by hour, with the minutes remaining unchanged.

A table of time differences relative to UTC is a useful tool.

9. History and evolution of the Complication

The need to indicate two times simultaneously arose with long-distance travel. The first marine clocks of the 18th century kept the time of the origin meridian for calculating Longitude. This onboard time was distinct from the ship’s local time.

The first pocket watches with dual time indication appeared in the second half of the 19th century. Their development coincided with the rise of the railway and the standardization of Time zones.

The dual time zone wristwatch developed from the 1950s onward. The rise of commercial aviation created demand among pilots and flight crews. Swiss watchmaking manufactures responded to this demand as early as that period.

The following decades saw the proliferation of display architectures. Coaxial second time zone hand, digital aperture / window, offset dial: each manufacturer explored different solutions. Contemporary production covers the full range of these approaches.

Today, the dual time zone watch is one of the most widespread travel complications. Its presence across all price segments — from industrial production to haute horlogerie — attests to its lasting functional relevance.