GMT watches or function to determine age
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What does "GMT" mean?
There are two completely different meanings:
A) “GMT” as a brand name
There were and are several manufacturers who have used the name GMT (often “GMT Watch Co” , “GMT SA” , “GMT Chronometer”, etc.), mostly for Swiss hand-wound or automatic watches from the 1950s to the 1980s.
B) “GMT” as a function
Many brands (Rolex, Omega, Seiko, Tudor, etc.) manufacture or manufactured so-called "GMT watches" — that is, watches with a second time zone (Greenwich Mean Time) .
Then "GMT" does not stand for the brand , but for the model or caliber .
If your watch bears the brand name "GMT".
Then it is most likely a Swiss or French trademark from the mid-20th century that sourced works from other manufacturers.
Typical data:
| Period | Factory manufacturer | Caliber / Notes | style |
|---|---|---|---|
| 1945–1960 | A. Schild (AS 1130 – 1132) | 15/17 rubies, small seconds | Classic hand-wound watch |
| 1960–1975 | ETA, FHF 72, Durowe, Felsa | Automatic with date, shock protection – “GMT Automatic” | stainless steel housing |
| 1975–1990 | ETA 2824, AS 2066 etc. | Automatic or quartz | Modern lettering “GMT Quartz” |
💡 Serial numbers of Swiss movements (e.g. ETA 2472, AS 1130) can be dated using the caliber serial lists.
– ETA numbers above 1,000,000 → after 1960
– A. Schild series from 500,000 → ca. 1950s
– FHF series over 1,500,000 → after 1970
If it is a "GMT function watch" (second time zone).
Then the dial will display, for example:
“Rolex GMT-Master”, “Omega GMT”, “Seiko GMT”, etc.
Example: Rolex GMT-Master
| Model | reference | Serial numbers | Years of construction |
|---|---|---|---|
| GMT-Master (Ref. 6542) | Caliber 1036 | Series 100,000–300,000 | 1954 – 1959 |
| GMT-Master 1675 | Caliber 1565 / 1575 | Series approx. 500,000–3,300,000 | 1960 – 1978 |
| GMT‑Master II 16710 etc. | Caliber 3185 / 3186 | Series from approximately R000000 (...) | 1989 – today |
The serial number (= movement + case serial number) allows conclusions to be drawn about the year of production (Rolex maintains detailed serial lists).
Example: Omega Seamaster GMT
| caliber | Construction period | Features |
|---|---|---|
| 1128 (automatic) | from 1990 | modified ETA 2892-A2 with GMT hand |
| 8605 (Co-Axial) | from 2011 | modern co-axial GMT version |
Example dating
| engraving | Factory | Description | Period |
|---|---|---|---|
| “GMT 15 Rubis Swiss Made” | A. Sign AS 1130 – No. 524 xxx | Manual winding, small seconds | ca. 1953–1955 |
| "GMT Automatic" | ETA 2472 – No. 1 243 000 | Automatic | ca. 1968–1970 |
| "GMT Quartz" | ETA 955.112 | Quartz caliber (7-digit number) | ca. 1990–2000 |
GMT watches are ideal for people who regularly work in different time zones or travel frequently. They allow you to keep track of up to two additional time zones simultaneously, in addition to your local time – a practical companion for businesspeople, frequent flyers, and globetrotters. But how did GMT watches get their world-famous name, how are they set, and what is the difference between "office" and "traveler" GMT watches? This article will tell you everything you need to know about the origins, functionality, and different types of these versatile watches.
The purpose of a GMT watch is to display Greenwich Mean Time (UTC) in addition to the local time. A simple glance at the often longer and differently colored 24-hour hand, which points to the 24-hour scale on the inner rehaut or bezel, is enough to identify the second—or third—time zone, while the traditional hour and minute hands are responsible for displaying the local time. Unlike the regular hour hand, which completes one revolution of the dial every 12 hours, the GMT hand completes one revolution every 24 hours—this will become important shortly.
The GMT bezel
This fact leads us to another important component of the GMT concept: the bezel. The 24-hour scale on the rehaut or bezel makes it possible to distinguish between day and night hours and to set a second or third time zone.
To better distinguish between day and night, many GMT bezels are therefore two-tone and can usually be rotated in both directions.
Source: swisswatches-magazine, AI, Google
