Dual time zone consists of having a second additional time hand to display a second time zone, i.e. the time in another city for example.

This additional hand is very often on 24h or on 12h but with a day and night display. There were sometimes some watches with double time zone on 12 o’clock but their production remained confidential because there could be confusion between the hours of the day and the night.

These watches are sometimes referred to by the generic term GMT which can be considered accurate, although these initials historically refer more to Greenwich Mean Time. With long distance travel much more common over the past several decades these watches have also been a constant success for several decades.

There is still a very basic way to have a double time zone which consists in simply having a notched bezel around an unmodified movement with a hand on 12 o’clock. Thus one will have a double time zone which will function however on 12h without differentiating the two parts of the day.