Understanding A GMT Timepiece And Its Function

What is a GMT (Greenwich Mean Time) watch and what does it do?

Came across a watch with 4 hands on the dial face? Or maybe you saw your friend wearing a watch that has 2 colours on the bezel? Those are GMT watches, short for Greenwich Mean Time, and contains the additional feature of being able to track multiple time zones simultaneously. In this article, we will break down everything you need to know about GMT watches.





Brief History of GMT

GMT complications were developed in the 1950s for pilots who would need to track multiple time zones. The most significant milestone was in 1954 when Rolex introduced the GMT-Master. Before the invention of GMT watches, pilots would often carry 2 watches to track different time zones. This made GMT-Master a sensation, capturing not only the attention of all the pilots but also travellers. Known for the classic blue and red bezel, the watch took off, increasing popularity of GMT.





How GMT Works

A GMT has 4 hands, a second, minute, hour and GMT hand. The GMT hand is usually longer than the rest and marked in a special colour. It serves to tell the time in a 24 hour format, usually marked on the watch face or bezel. This goes one step further by adjusting the 24-hour marked bezel to track an additional time zone. This one is trickier as it took me some time to get adjusted to it as well. Set the 24 hour bezel forwards or backwards with reference to your base time zone (minutes, hour and seconds). You can then read the time of the 3 hands, with reference to the 24 hour bezel instead of the usual 12 markers on the watch face. Setting the GMT hand differs between 2 different types of GMT watches; office/callers and true/flyer/travellers.





The Caller/Office GMT

The caller GMT is classified by the hour GMT hand operates in relation with the local time. Meaning, when you adjust the local time (3 hands) to another time zone, the GMT hand will move too, and it won’t be tracking the original time it was set to. You have to manually set the GMT hand back to the desired secondary time zone.

To set the time on a callers GMT, unscrew the crown and pull it out fully to adjust the local time. Afterwards push it back in before pulling the crown out softly by 1 notch moving the crown clockwise adjusts the GMT individually, moving in the opposite should adjust the date. Push the crown back in and screw it in place once done.

The idea behind the name caller or office GMT is that usually, you are in a country and do not travel much, the purpose of the GMT is just to track another time zone, per say your boss or family member. The term caller or office refer to the idea that it is ideal for those who do not frequently travel.

Hope the explanation was clear but if you are still confused with the concept. Come down to Gnomonwatches or any watch shop and gets hands on with a caller GMT to get a better understanding. Some suggestions you can find at Gnomonwatches will be stated below.





The True/Traveller/Flyers GMT

The True GMT is classified by the GMT hand functioning separately from the local time. The simplest way to identify this watch is when you are able to adjust only the hour hand by increments of 1 hour.

To set the time on a True GMT, unscrew the crown and pull it out all the way. Adjust the reference time (time which you want to track) of the GMT hand first. After doing so, push the crown back in and pull the crown by one notch. You will be able to adjust the local time and date via the hour hand by doing so. Push the crown back in and screw it in place once done.

Following the logic behind the name, the Traveller/Flyer/True GMT are ideally for frequent flyers who move from time zones and would like to track multiple time zones. Think of a pilot who goes from one flight to another, he would need to adjust the local time of his watch each time he gets to another country but would like the GMT hand to keep track of the same timing. Adjusting the local time with a caller GMT would result in the GMT hand moving as well, thus needing to adjust the GMT back afterwards to ensuring time tracking.

The term “True” is not to say that the caller GMT is “untrue” in any sense. The term “True” is often quoted by collectors in the watch community as they believe that if the GMT was made for pilots to track different time zone, the best or “true” approach to doing so was the Traveller GMT.





Implications

When adding a GMT into your collection, one can look at their lifestyle to buy GMT that suits them. A traveller GMT movement itself usually cost more than a caller GMT as well. Choosing a caller GMT can be more budget friendly as well whilst still serving to track multiple time zones. At the root of it all, we always recommend buyers to always go for a watch that they like, as both functions as a watch that tracks another time zone. Such considerations should be secondary as we believe a watch’s value is always dictated by its buyer.



Top 3 Personal Picks


Hitori Yoshino Night Star Indigo GMT – Ltd Ed 100pcs (Caller)

My first impression of the Yoshino Night Star Indigo GMT, limited to just 100 pieces was its unique and captivating colour scheme, with the enchanting night star dial as the standout feature. The dial captures the mystical charm of a starlit night, true to its oriental roots, while maintaining a minimalistic aesthetic with its beze-less case and clean design. This allows each element of the watch, from the applied indices to the well-finished hands and stunning gradient dial, to shine without distraction. Every detail is painstakingly well-executed, creating a harmonious balance between elegance and functionality.

This GMT complication, accented by a contrasting 24-hour hand, allows for easy tracking of a second time zone, making it both a visual masterpiece and a practical tool for the modern traveller. Under the hood, the Yoshino Night Star is powered by the ever-reliable NH34 automatic movement, offering 41 hours of power reserve. This movement is displayed through the exhibition screwed-down case back, where the well-finished mechanics of the watch are revealed—a feature collectors will appreciate for its craftsmanship and transparency. The 39.6mm case size strikes a fine balance between presence and wearability, appealing to a range of wrist sizes, while the 100-meter water resistance adds a robust functionality that enhances its versatility.





Prospex 200M Automatic Diver Scuba GMT Black Ref. SBEJ011 (Caller)

At first glance, this watch captures the elegance of the Rolex Sub, but as you dive past the surface, you come to view the Seiko DNA weaved into this watch. This Seiko Caller GMT is powered by the ever-reliable Seiko 6R54 movement with 72-hours of power reserve, meaning when u put it down on Friday for the weekend, it will still be running on Monday morning. With a screwed down case back featuring Seiko diver’s iconic tsunami logo, this watch is built to last with a 200m water resistance.

Along with the golden GMT hand, the watch is well finished, featuring the iconic Zaratsu polishing, Seiko’s specialised polishing skill, makes for a well-defined case. With lumibrite-coated markers, Seiko craftmanship is showcased extremely well in this piece, living up to it’s reputation. Overall, it is a strong addition to any collection suiting as an all rounder sports watch, reliable and well made, it is a steal for it’s price.




Mido Ocean Star GMT Special Edition Rose Gold (Ref.M026.629.22.051.00) (True)

My first impression of the Mido Ocean Star GMT was how sharp it was and the elegant aesthetic. Diving into the Mido Ocean Star, which is a true GMT, the watch stands out due to its mix of luxury aesthetics, practical functionality, and Mido’s longstanding history of producing quality timepieces. This model’s rose gold PVD accents immediately grab attention, offering a sense of luxury that complements the robust functionality of a GMT diver. The combination of the black dial and bezel with the rose gold tones gives the watch a sophisticated look that can transition effortlessly from diving environments to more formal settings, a versatility that collectors often seek.

Under the hood, the Caliber 80 automatic movement provides a noteworthy 80-hour power reserve, meaning that when you put down the watch on Friday, its still running on Monday morning. In terms of build quality, the watch's 200m water resistance reinforces its dive watch roots, while the 42.5mm case size strikes a balance between a commanding wrist presence and wearability for a variety of wrist sizes. For collectors who appreciate a blend of luxurious design with tool-watch functionality, the Mido Ocean Star GMT Rose Gold offers a compelling package that brings together elegance, practicality, and Swiss precision at an accessible price point.