GMT stands for 'Greenwich Mean Time' which is the clock time at the Royal Observatory in Greenwich London.
It is the same all year round and is not affected by the Summer Time or Daylight Saving Time. At 12:00 noon at Greenwich, the sun is at its highest point exactly above the Prime Meridian.
GMT is still generally used as the standard time against which all other time zones in the world are referenced. A watch is basically made to tell time. Any other function on a watch is called a ‘complication’.
Even adding a simple ‘date’ on a watch involves the addition of very complicated gears and linkages. So the GMT-function is also a ‘complication’.
One day, people suddenly able to fly across oceans and time zones in a single day. The pilots flying these huge planes and those long haul routes needed a way to keep track of time, both local time and the time in the place from where they departed. Pan Am airlines approached Rolex and asked them to produce a watch for their pilots. Then the GMT-Master was born, a watch that could track two time zones simultaneously.
But how do we use it on a watch while travelling?
GMT watches usually have a fourth hand that can be independently adjusted.
Step 1: Determine which time zone you would like to set the GMT hand to. For this example, let’s say we’re living in Los Angeles (GMT-7) , and want to set the second time zone to Tokyo, Japan (GMT+9).
Step 2: We would set the time of the watch to 12AM.
Step 3: At the time of writing, Los Angeles is currently in Daylight Savings Time which means that Tokyo is only 16 hours ahead. Knowing this, we would point our GMT hand to the “16” on the chapter ring. You can adjust the GMT hand by pulling out the crown to its first position and turning it counter-clockwise.
Step 4: Then, we would set the time and date of our watch. You’ll notice that the fourth hand will rotate in unison with the hour and minute hand, and is constantly set 16 hours ahead of the current time.