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1. Definition and purpose of the complication
The dual time zone (GMT) watch simultaneously displays the time of two distinct places. One of the two indications represents the 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 is fundamentally different 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 document.
2. Time zones: astronomical basis
The earth completes a full rotation on itself in twenty-four hours. This movement produces the Vibration of day and night. It is at the origin of the concept of Time zone.
The earth’s surface is divided into twenty-four theoretical Time zones. Each zone corresponds to fifteen degrees of Longitude. The legal time differs by a full hour between two adjacent zones.
This division is an international convention established in 1884. The Washington Conference chose the Greenwich meridian as the reference. This meridian defines the zero Longitude of the planet.
In practice, the boundaries of the zones do not follow the meridians. Political and geographical divisions deviate them considerably. A single country can cover several zones with only 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) — UTC — is the worldwide 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, from 1972 onward, be considered inaccurate.
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 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 scale visible on the dial’s inner ring or the Case Bezel. A darker zone of this scale is generally present to identify nighttime hours (between 6 pm and 6 am).
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 scale. Sometimes, the relevant area of the Dial is indicated by a shaded 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 occurs on a twelve-hour cycle. Either at the center, as in the previous example, or on a small autonomous subdial that displays the second time zone with its own hands. Its legibility is lower compared to a center display. This type of display generally rotates over twelve hours. As it does not distinguish between daytime and nighttime hours, it is generally supplemented by a day/night indicator with aperture, thus avoiding any unfortunate confusion.
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4.3 The time aperture / window
Some calibers display the second time zone in a digital aperture / 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 with 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 the multi-position crown is a precise mechanical challenge. Each crown position must control a distinct function and involves a construction specific to this complication.
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6. La correction par poussoir(s)
La correction par poussoir est plus rapide que la correction par couronne. Un poussoir dans la carrure fait avancer l’aiguille d’heure du second fuseau horaire d’une heure à chaque pression. Un second poussoir permet le recul.
Le mécanisme interne repose sur un dispositif à cliquet et à étoile. Chaque pression fait avancer ou reculer l’étoile d’un pas. L’étoile déplace l’aiguille d’heure du second fuseau horaire d’exactement une heure.
La possibilité de corriger dans les deux sens est précieuse. Elle facilite les voyages est-ouest autant que les voyages ouest-est. Elle évite de devoir faire le tour complet des 24 heures pour un recul d’une heure.
Un sautoir immobilise l’étoile entre deux pressions successives. Ce ressort garantit que l’aiguille d’heure du second fuseau horaire s’arrête précisément à chaque heure.
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7. Le changement d’heure saisonnier
Le changement d’heure saisonnier impose une correction manuelle de la montre à deux fuseaux. Ce changement est d’une heure, dans un sens ou dans l’autre, selon le calendrier de chaque pays.
À quelques rares exceptions, une montre mécanique n’intègre pas ce changement automatiquement. L’utilisateur doit corriger l’un ou l’autre des deux fuseaux affichés. Le mécanisme de saut d’heure rend cette correction rapide et précise.
La situation se complique lorsque les deux fuseaux appartiennent à des régions aux calendriers différents. Le jour du changement d’heure varie selon les pays. Certains pays, comme le Japon, n’appliquent aucun changement saisonnier.
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 offsets 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 railways and the standardization of time zones.
The dual time zone (GMT) 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 from that period onward.
The following decades saw a 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 (GMT) watch is one of the most widespread travel complications. Its presence across all price segments — from industrial production to haute horlogerie — reflects its lasting functional relevance.
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