World Clock - Most efficient

A world clock is a clock that displays the time for various cities around the world. The display can take various forms:

The clock face can incorporate multiple round analog clocks with moving hands or multiple digital clocks with numeric readouts, with each clock being labeled with the name of a major city or time zone in the world. The World Clock in Alexanderplatz displays 146 cities in all 24 time zones on its head.

It could also be a picture map of the world with embedded analog or digital time displays.

A moving circular map of the world, rotating inside a stationary 24-hour dial ring. Alternatively, the disc can be stationary and the ring moving.

Light projection onto a map representing daytime, used in the Geochron, a brand of a particular form of the world clock.