As far as watch complications go, a lot of them are reminders of earlier times. They were seen as novelties at the time but they’ve since become obsolete. Some of them are even completely pointless (I’m looking at you, moon phase).
Watchmakers still use them because they’ve become part of the design.
The GMT complication, however, is still very useful to display two separate time zones at once.
Not only does this come in handy for people who travel a lot, it also looks badass.
So, what is a GMT watch and how do you use it?