a Greenwich Mean Time or (GMT) watch can simultaneously display two different time zones. A GMT uses a 24-hour second-time zone display (whereas a “Dual Time” watch uses a 12-hour second-time display). Greenwich Mean Time is also the mean (average) solar time at the Greenwich Meridian or Prime Meridian, 0 degrees longitude. The time displayed at the Royal Observatory in Greenwich, London, is always GMT.

a flyer GMT, also referred to as a “true GMT” or “traveler GMT” allows you to independently set the local 12-hour hand.

a caller GMT, also referred to as an “office GMT” allows you to independently set the GMT/24-hour hand.