How do I set a GMT watch?

There is no single procedure, and that is the reason this question keeps being asked. Two watches can carry identical dials and be set in opposite orders, because the manufacturer chose a different hand to make independent. Work out which type you own before you touch anything else.

The vocabulary is settled even if the marketing is not. A true GMT, also called a traveler or flyer GMT, lets you jump the local hour hand. A caller GMT, also called an office GMT, lets you set the 24-hour hand. Both display two zones. Only the first is convenient to travel with, and the background is in the guide to GMT and world time watches.

Work out which kind you have

Unscrew the crown if it screws down, then pull it out one click to its middle position and turn it slowly while watching the dial.

• The main hour hand hops in one-hour steps, the minute hand stays still, and the 24-hour hand does not move: you have a true GMT.
• The 24-hour hand steps round on its own while everything else stays put: you have a caller GMT.
• Only the date advances: your watch puts the second zone somewhere else, usually in a different crown position or a second click. Keep going.

On some watches the date quickset shares the middle position, one direction of turn per function. Turning slowly and watching, rather than turning and hoping, is the whole technique.

Setting a true GMT, step by step

The logic is: build the whole watch on your reference zone, then move only the local hand.

1. Wind the watch if it has stopped, 30 to 40 turns, so the hands move under proper torque.
2. If the bezel rotates, return it to its zero position, with 24 (or 0) at the top. Otherwise everything you set afterwards will be offset.
3. Pull the crown to its furthest position. On most modern calibers the seconds hand stops, which is hacking, so you can synchronize to a time signal.
4. Set your reference zone. In this position the hour, minute and 24-hour hands move together. Bring the 24-hour hand to your home time on the 24-hour scale, and use it, not the hour hand, to decide whether you are in the morning or the evening.
5. Push the crown to the middle position and set the date if the date is wrong, avoiding the hours either side of midnight.
6. Still in the middle position, jump the local hour hand forward or back to the time where you are. The date follows it across midnight on most calibers, which is exactly the behavior you want after a flight.
7. Push the crown home, and screw it down if it is a screw-down crown.

From then on, travel is one operation: click the hour hand to local time. The minutes are never disturbed, the seconds keep running, and your home zone stays readable on the 24-hour hand.

Setting a caller GMT, step by step

The order is reversed, because the 24-hour hand here is geared to the main hands and will be dragged along when you set the time.

1. Wind the watch.
2. Set the bezel to zero if it rotates.
3. Pull the crown fully out and set local time, the zone you are actually in. Check the 24-hour hand to confirm you are on the correct half of the day: if it is 21:00 the 24-hour hand should point at 21, not 09.
4. Push the crown to the middle position and set the date.
5. In the same position, step the 24-hour hand to the second zone. Many calibers only advance it forwards, so going back an hour can mean 23 clicks forwards.
6. Push the crown home and screw it down.

Because the second zone is set last, you never have to redo it after a time correction, provided you only correct minutes. Move the hour hand and you have moved the GMT hand with it, so check both.

Using the bezel for a third zone

A rotating 24-hour bezel turns a two-zone watch into a three-zone one, at no mechanical cost. The 24-hour hand keeps pointing where it always did. You move the scale underneath it.

The rule: rotate the bezel counterclockwise to add hours, clockwise to subtract them, one hour per marker. On a 24-marker bezel that is 15 degrees per hour. If the 24-hour hand is set to London and you want Dubai, which is three hours ahead in winter, rotate the bezel three markers counterclockwise and read the hand against the bezel.

Three practical notes. First, verify with a zone you know rather than trusting the direction rule, because it is easy to reverse. Second, once the bezel is offset it no longer reads the 24-hour hand's own zone, so you have swapped one zone for another unless the dial also carries a fixed 24-hour ring. Third, GMT bezels rotate both ways by design, unlike a diver's bezel, which ISO 6425 requires to be unidirectional. If your bezel only turns one way, you are probably holding a dive watch with a 24-hour hand, and the third zone has to be counted the long way round.

Daylight saving and awkward offsets

Twice a year the arithmetic changes underneath you, and the two types of watch handle it differently. On a true GMT, moving your own zone is one click of the hour hand. On a caller, both hands may need attention: the main hands for your zone, and the 24-hour hand for the other one if that country shifts on a different date. Southern hemisphere zones move in the opposite direction from northern ones, and some countries do not shift at all.

Half-hour and quarter-hour offsets are the genuine limitation. India sits at UTC+05:30, Nepal at UTC+05:45, and parts of Australia at UTC+09:30. An hour-stepping hand cannot show them. If the bezel turns freely rather than clicking in one-hour steps, you can offset it by half a marker and read the half hour zone that way. Otherwise you add the thirty minutes in your head, or you buy a world timer, which handles cities rather than offsets. The trade-off between the two complications is set out in GMT versus world timer.

What to check before you buy

If you have not bought yet and you actually travel, the type matters more than the brand. Look in the specification or the manual for the behavior rather than the label: "independently adjustable hour hand" or "local hour hand jumps in one-hour increments" means a true GMT, while "independently adjustable 24-hour hand" or "second time zone set in crown position 1" means a caller. Listings that say only "GMT function" are usually caller, because caller is cheaper to build. The reasons a second zone is worth having in the first place are in what a GMT watch is for.

Whichever you own, keep the manual. Crown positions are not standardized, the number of clicks varies between two and three, and the direction of travel for the date quickset differs by caliber. General setting practice, including hacking and screw-down crowns, is covered in setting and adjusting your watch. Everything else on a GMT is just deciding which zone deserves the hand you can read at a glance.