GMT and world time watches

A GMT watch adds a fourth hand that goes round the dial once every 24 hours instead of once every 12, read against a 24-hour scale. Point it at a second time zone and you can see, at a glance, both what time it is where you are and what time it is somewhere else, including whether that somewhere else is in the middle of its night.

The 24-hour scale is the whole point. A normal hour hand cannot distinguish 3am from 3pm, which is exactly the information you need before you telephone anyone. That is also why the second hour hand is usually a different color or shape, and why the scale runs 0 to 23 or 1 to 24 around the dial edge, a chapter ring or a bezel.

True GMT: the traveler's version

Pull the crown to its intermediate position and turn it, and the local hour hand hops forward or back one hour at a time with an audible click. The minute hand does not move. The seconds hand does not stop. The 24-hour hand stays exactly where it is, still pointing at your home zone. On most such watches the date changes with the local hour hand as it passes midnight in either direction.

You land in Singapore, click the hour hand round, and the watch is on local time with your home zone still readable and your minutes never disturbed. Rolex introduced this behavior to the GMT line with the GMT-Master II in the 1980s, and it is now the specification serious travellers look for.

The cost is mechanical: an independent hour wheel with its own jumper and detent adds parts, height and complexity, so true GMTs sit higher up the price ladder.

Caller GMT: the version for staying put

Here the arrangement is reversed. The local time hands work exactly as on any watch, and a crown position lets you set the 24-hour hand independently, usually in one-hour steps.

That suits somebody who lives in one place and needs to know the time somewhere else: a colleague in New York, family in Sydney, a market that opens in Tokyo. You set the second hand once and forget about it, except when daylight saving moves at either end.

Take a caller GMT abroad and it is awkward. To change your local time you have to move the main hands, which means the 24-hour hand moves too (it is geared to the going train), so you end up resetting both, and you disturb the minutes to do it.

Telling them apart before you buy

Manufacturers are not always explicit, so read the specification for behavior rather than names.

If a listing simply says "GMT function" with no detail, assume caller until proven otherwise, because caller is far cheaper to build.

The bezel: a third time zone for free

If the 24-hour scale is on a rotating bezel rather than printed on the dial, you get a third zone at no mechanical cost. Rotate the bezel by the difference between the zone the 24-hour hand is set to and the zone you want, and read that hand against the moved scale. Twelve clicks counterclockwise, for example, and the hand now reads a zone 12 hours away.

Two practical notes. This works only if the bezel is bidirectional, because a zone can be ahead or behind you. And a bidirectional bezel is exactly what a dive watch must not have: ISO 6425 requires a dive bezel to rotate one way only, so an accidental knock can never make your remaining air look longer than it is. That difference in intent is covered in our guide to dive watches and ISO 6425. A GMT bezel is an information display. A dive bezel is a safety device.

Most GMT bezels click in 24 positions, one per hour. A few are friction-fitted and can be set to any position, which is the only way to handle a half-hour offset zone.

Setting procedures

Both procedures assume the watch is running and wound, and both depend on doing the steps in order.

Setting a true GMT

1. Unscrew the crown if it screws down, and pull it to the furthest position. The seconds hand should stop.
2. Turn the hands until the 24-hour hand reads your home or reference time on the 24-hour scale. The local hour hand and minute hand move with it, and that is fine. Watch the 24-hour hand, not the main hands.
3. Check you are in the right half of the day. If the 24-hour hand is pointing at 09 when your reference zone is at 9pm, run the hands round another 12 hours.
4. Set the minutes precisely against a time signal, then push the crown to the intermediate position.
5. Click the local hour hand forward or back until it shows your current local time. Each click is one hour. The date will roll over as the hand passes midnight, which is normally how you set the date too.
6. Push the crown home and screw it down if applicable.
7. If you are using a bezel for a third zone, rotate it last.

From then on, traveling means step 5 only.

Setting a caller GMT

1. Unscrew the crown if necessary and pull to the furthest position. Set your local time in the ordinary way, including the correct half of the day, and set the minutes to a time signal.
2. Push the crown to the intermediate position.
3. Turn the crown to move the 24-hour hand to the second zone you want. Direction and number of clicks depend on the offset: forward for zones ahead of you, back for zones behind.
4. Set the date, usually in the same crown position but turning the other way. Check your manual.
5. Push the crown home.

World timers

A world timer shows every zone at once. A ring of 24 city names, one per zone, sits around the dial against a 24-hour ring. Align the city of your current zone with the local time and every other city can be read directly against the 24-hour ring. The concept was developed by the Geneva watchmaker Louis Cottier in the 1930s and supplied to several manufacturers, and the layout has barely changed since.

Setting varies by design. The common arrangement uses a pusher or a second crown to index the city ring one city (one hour) at a time, with the 24-hour ring driven by the movement so it stays in step with your local time. On some models you index the hour hand instead and the city ring stays put.

The limitations are worth knowing before you buy one.

• Half-hour and quarter-hour zones do not fit. India is UTC+05:30, Nepal is UTC+05:45, the Chatham Islands are UTC+12:45, and Iran, Newfoundland and the Marquesas all sit off the hour. A 24-position ring cannot represent them, so those places are either omitted or shown wrong.
• Daylight saving makes the ring wrong for part of the year. The city ring is a fixed engraving of standard-time offsets. When London goes to British Summer Time, or New York to Eastern Daylight Time, the ring is an hour out for those cities until the clocks go back. Some designs print a second, summer-time set of cities, or mark them, but nothing can make an engraved ring track legislation.
• Zone rules change. Countries move zones and abolish daylight saving, so an older city ring may be quietly wrong about somewhere today.

None of that makes a world timer useless. It makes it a very good approximation with footnotes you have to remember.

UTC or GMT?

Strictly, the modern civil time standard is UTC, Coordinated Universal Time, an atomic timescale kept close to the earth's rotation by the occasional leap second. GMT, Greenwich Mean Time, is mean solar time on the prime meridian at the Royal Observatory, Greenwich, and was superseded as a formal timescale decades ago. For any purpose a wristwatch serves they are the same thing, differing by well under a second.

The watch industry says GMT for historical reasons. The complication was made famous by the Rolex GMT-Master of the mid-1950s, developed in association with Pan American World Airways for pilots crossing multiple zones, and the name stuck to the category the way "Hoover" stuck to vacuum cleaners. Some brands print UTC on the dial instead, meaning exactly the same function.

Who each one suits

• You fly regularly: a true GMT. Set home time on the 24-hour hand and forget it. Each new zone is a few clicks and nothing else is disturbed.
• You work across zones from one desk: a caller GMT is genuinely better value, and the resetting inconvenience never arises because you never move.
• You want to see many zones at once: a world timer, understanding that it is an approximation for half the year.
• You want a third zone occasionally: any GMT with a bidirectional 24-hour bezel.

The broader question of which functions earn their place on a watch is covered in our complications overview.