A watch for two time zones

Every long-haul traveller knows the moment. It is late afternoon in Cape Town, and you are trying to work out whether it is too late to call home. A GMT watch answers that at a glance, which is why it is one of the most useful complications in watchmaking.

How it works

A GMT watch adds a fourth hand that rotates once every 24 hours rather than every 12. Read against a 24-hour scale on the dial or bezel, it shows a second time zone, and makes clear whether it is day or night there. Many GMT watches use a rotating bezel, so you can track a third zone as well.

The complication is closely associated with the jet age. Rolex's GMT-Master, introduced in the 1950s, is often linked with Pan Am's long-haul pilots, and its two-colour bezel, red and blue, has been imitated ever since.

Two kinds of GMT

The detail that matters most when choosing one is which hand you can adjust independently.

• A traveller's GMT lets you jump the local hour hand forward or back in one-hour steps without stopping the watch. When you land, you change the local time in seconds and the 24-hour hand keeps home time. This is the more convenient choice for frequent travellers.
• An office GMT (sometimes called a caller GMT) lets you set the 24-hour hand independently instead. It is ideal for keeping an eye on a colleague's time zone from your desk, and slightly less convenient on arrival.

Neither is better. They simply suit different lives.

Choosing one for a journey

A GMT makes the most sense on journeys that genuinely cross the world, from Europe to Japan, South Africa or the Indian Ocean. Look for:

• Legibility. A clearly distinguishable 24-hour hand, visible in a dark cabin.
• Water resistance suited to swimming, if the trip includes the sea.
• Comfort. It will be on your wrist for a twenty-hour door-to-door day, so a bracelet with fine adjustment or a soft strap matters.

Set it before you leave, then stop thinking about time. Knowing that it is breakfast time at home is enough.