When shopping for a dual-time or GMT watch, consider the variations that are possible. First, not all GMT watches are true dual-time machines, and not all dual-time watches are GMTs.
What Makes a GMT vs Second Time Zone
Classically (think Rolex 6542, 1675, 16750, 16700), the GMT is a watch that offers a 12-hour hand and a secondary mechanism to discern – momentarily – a second time zone.
These watches, which usually feature a bidirectional bezel, allow the user to “offset” from the time on the dial and calculate a local time mathematically. On the original Rolex GMT-Master, the “24-hour” hand would act as an AM/PM indicator permanently synched with the local hour hand at center, and the watch would be set to Greenwich Mean Time; the 24-hour hand could not be set independently.
This is useful for pilots flying into many airports, but it doesn’t allow convenient AND constant reference to a second time zone.
Later, with the GMT-Master II and similar watches such as the Carl F. Butcherer Patravi TravelTec, and the current Breitling Chronomat GMT, the 24-hour hand can be set independently of the shorter “local” hour hand. However, the 24-hour bezel remains, so these watches can be used in the same fashion as the original Rolex GMT-Master, but the addition of an independent second time zone means that THREE time zones can be referenced momentarily on these watches.
What to Know When You Buy
When considering a dual-time watch or GMT, know whether the 24-hour hand can be set independently to a second time zone. Moreover, if the watch lack as 24-hour hand, check to see if the second time zone is accompanied by an AM/PM or night/day indicator. If not, the watch cannot reliably be used to discern the time of day in a remote reference area of the world. These watches may be “dual-times,” but they cannot be considered GMT watches in the technical sense, because Greenwich Mean Time respects a 24-hour format.
As always, observe the golden rule of collector passion; once you have the tools to make an informed purchase decision, discard all technical minutia and buy what you love.