Originally Posted by culverwood
There is a third hand which revolves on a 24 hour basis which tells the time against a second time zone while the normal hands keep home time (or vice versa) the second time zone can vary from country to country hence the need for rotation.
On Rolex GMT-Master IIs (both modern and vintage), the 24 hour hand is independent from the hour hand, so that you can track 3 timezones at once. 1st: the hour hand. 2nd: the 24hour hand against the dial. 3rd: the 24 hour hand against the bezel. On this vintage GMT-Master (note, no II
), the 24 hour hand is not independent from the hour hand. So it really only tracks day/night. But with the rotating bezel, you can keep track of the second timezone.