GMT Watch Guide: Tracking Multiple Time Zones

GMT watches were designed for pilots and international travelers who needed to track time in two places simultaneously. Today, they remain among the most practical complications in watchmaking—and among the most misunderstood. Here’s how GMT watches actually work and why the distinction between “true GMT” and “office GMT” matters for how you’ll use the watch.

Understanding the GMT Function

A GMT watch displays a second time zone using an additional hour hand that completes one rotation every 24 hours instead of 12. This 24-hour hand points to a scale (usually on the bezel or rehaut) marked 0-24, indicating whether the referenced time zone is in AM or PM.

The term “GMT” refers to Greenwich Mean Time, the time at the Prime Meridian in London. Historically, pilots and navigators used GMT as a reference point when crossing time zones. Today, most GMT watches function as dual-time-zone displays rather than specifically referencing Greenwich.

True GMT vs. Caller/Office GMT

Not all GMT movements function identically. Understanding the difference affects how you’ll set and use the watch:

True GMT (Traveler’s GMT): The main hour hand can be set independently in one-hour increments. When you land in a new time zone, you advance or retard the main hour hand without affecting the 24-hour hand or the running of the watch. The 24-hour hand continues tracking your home time. This design prioritizes the local time (where you are) while the GMT hand tracks reference time (typically home).

True GMT watches include the Rolex GMT-Master II (post-2018), Tudor Black Bay GMT, Grand Seiko SBGJ, and Omega Seamaster Aqua Terra GMT.

Caller/Office GMT: The 24-hour hand adjusts independently while the main time remains fixed. When you want to reference a different time zone, you position the GMT hand accordingly. The local time stays on the main dial; you adjust the secondary display for your reference zone.

This design works well for people who stay in one location but need to reference other time zones—hence “caller” (calling someone in another zone) or “office” (desk-bound professionals). Examples include older Rolex GMT-Masters, many Seiko GMT models, and most affordable GMT watches.

The Rotating Bezel

Many GMT watches feature a rotating bezel marked with a 24-hour scale. This bezel can track a third time zone:

Set the main hands to local time
The 24-hour hand indicates home time on the fixed reference (often the rehaut)
Rotate the bezel to align with the 24-hour hand for a third reference

This third-zone capability comes free with any bezel-equipped GMT watch. However, reading the third zone against a moving bezel is less precise than dedicated hand displays.

24-Hour Display Reading

Reading a 24-hour scale requires adjustment from standard time-telling:

1-12: Morning hours (AM). 0600 is 6:00 AM.

13-24: Afternoon and evening hours (PM). 1800 is 6:00 PM, 2200 is 10:00 PM.

Most people adapt quickly after a few days of wear. The key is remembering that the 24-hour hand moves half as fast as the regular hour hand—pointing to 12 at noon and midnight, not twice daily.

Setting a GMT Watch

For True GMT watches:

Pull the crown to the first position
Rotate to advance/retard the main hour hand in one-hour increments
Leave the 24-hour hand tracking your home time
Push crown in to resume normal operation

For Caller GMT watches:

Pull the crown to the first position
Rotate to position the 24-hour hand to your reference time zone
The main hands remain at local time
Push crown in to resume normal operation

Practical Applications

Business travelers: True GMT shines here. Land in Tokyo, adjust the hour hand forward, and immediately display local time while your GMT hand shows what time it is back home. No mental math required.

Remote workers with international clients: Caller GMT works perfectly. Keep local time on the main dial, set the GMT hand to your primary client’s time zone, and glance down to know if calling now is reasonable.

Pilots and aviation professionals: The original GMT use case. Cockpit procedures use UTC (essentially GMT), so pilots set their 24-hour hand to UTC while the main hands show local time.

Stock traders and financial professionals: Market hours in different zones matter. A GMT watch tracking New York, London, or Tokyo market openings proves more useful than it sounds.

GMT vs. World Time vs. Dual Time

Related complications offer different approaches to multiple time zones:

GMT: Two time zones displayed simultaneously. Most practical for tracking one reference zone while living in another.

World Time: Displays all 24 major time zones on a rotating city disc. Beautiful but less immediately readable than GMT. The Patek Philippe 5131 exemplifies the type.

Dual Time: A second hour hand or subdial shows a second zone without the 24-hour scale. You must remember whether the reference zone is AM or PM. Simpler mechanism but less information at a glance.

Buying Recommendations by Need

Frequent travelers crossing many zones: True GMT with independently adjustable hour hand. Rolex GMT-Master II, Tudor Black Bay GMT, or Longines Spirit Zulu Time.

Occasional travelers or reference use: Caller GMT works fine and costs less. Seiko Prospex GMT, Orient Star GMT, or vintage Rolex GMT-Masters.

Style-conscious professionals: Dressier GMT options from Grand Seiko, Omega Aqua Terra, or Jaeger-LeCoultre Master Geographic offer versatility for boardroom to airport.

Budget-conscious buyers: Seiko 5 GMT models deliver the complication under $500. Orient and Tissot offer quality options under $1,000.

Living with a GMT Watch

Daily wear teaches you to read the 24-hour scale instantly. After a few weeks, glancing at the GMT hand tells you home time without conscious calculation. The complication becomes genuinely useful rather than decorative.

For international professionals, travelers, or anyone with regular contact across time zones, a GMT watch solves real problems. For others, it’s a pleasant complication that looks good and occasionally proves handy. Either way, understanding how your specific GMT movement works ensures you get the most from the watch.