World timer

A world timer shows the time in all 24 standard hour zones at once. A ring of city names, one city standing for each zone, sits around the dial next to a 24 hour ring marked with the hours and usually shaded to separate day from night. Local time is shown conventionally by the hands, and the time anywhere else is read by looking at the hour on the 24 hour ring opposite that city's name.

Two rings do the work and one hand is shared. The 24 hour ring is geared to the hour wheel through a two to one reduction, so it turns once a day while the hour hand turns twice. The city ring is indexed in 24 positions, 15 degrees apart, one for each whole hour of longitude. The minutes belong to nobody in particular: a single minute hand serves every zone at once, which is the structural fact behind everything the complication can and cannot do.

How it works

Setting the watch to a new location steps the local hour hand and the city ring together, usually through a pusher or an extra crown position, so that your current reference city is brought to the marker at the top of the dial and every other city falls into place behind it. The minute and seconds hands are untouched, which is the same virtue a traveler GMT has.

The layout descends from the work of the Geneva watchmaker Louis Cottier, who developed a practical world time mechanism in 1931 and supplied it to Patek Philippe, Vacheron Constantin and Rolex among others. Most modern world timers are variations on the same idea. What it solved was arithmetic: the standard hour zones that spread across the world after the 1884 International Meridian Conference are easy to calculate once and tedious to calculate repeatedly.

Limits worth knowing

The mechanism assumes 24 zones spaced exactly one hour apart, which the world is not. India and Sri Lanka run 30 minutes off the hour, as do Iran, Afghanistan at 4:30, Myanmar, Newfoundland and central Australia. Nepal sits 45 minutes off, at UTC+5:45 since 1986, and so do the Chatham Islands at UTC+12:45. Counting daylight saving, closer to forty distinct offsets are in use worldwide against the 24 a city ring can hold.

Daylight saving is not tracked either. For much of the year some of the cities on the ring are an hour out from what the dial says, and you have to remember which, since the ring cannot know. Zones also change politically: Samoa skipped 30 December 2011 entirely to move west of the date line, and Russia abandoned seasonal clock changes in 2011, shifting its offsets again in 2014. A printed city ring records the state of the world on the day it was engraved.

Variations you will meet

• Fixed cities, rotating hours. The classic Cottier layout, used by most mechanical world timers.
• Rotating cities. The city ring turns instead, bringing your reference city to a fixed marker.
• Bezel world timers. A rotating 24 hour bezel printed with city names and no extra mechanism at all. It is a lookup table you align by hand rather than a complication, and it is honest work at a low price.
• Quartz, radio or satellite synchronized. Electronic world timers select a city and move the hands or display to match. Because the rules live in firmware they can handle daylight saving and be updated when a country changes its offset, which no engraved ring can do.

That last point is the honest technical comparison: an inexpensive quartz world timer is more correct about the world than a mechanical one at any price. What the mechanical version offers is the mechanism, not the coverage.

World timer or GMT

A GMT watch shows one additional time zone with a fourth hand against a 24 hour scale. It is easier to read at a glance, easier to set, and better suited to somebody who travels between two places or wants to keep an eye on home. A world timer shows everywhere at once, which is a reference tool rather than a travel one.

It is also busier. Fitting 24 city names around the edge of a 40 mm dial leaves each one about a millimeter of type, so world timers run large and their dials are crowded, a trade our guide to dials, hands and legibility takes seriously. Nor is a world timer the same as a dual-time watch, which shows a second zone on a subdial or in an aperture. For the full comparison see GMT against world timer and our guide to GMT and world time watches; if you fly rather than telephone, how to set a GMT watch explains why the traveler version usually wins.