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Digital clocks display a numeric representation of time.Two numeric display formats are commonly used on digital clocks: 24-hour notation and 12-hour notation.Independently, the Chinese developed their own advanced water clocks（水鐘）in 725 A. Some water clock designs were developed independently and some knowledge was transferred through the spread of trade.
Some predecessors to the modern clock may be considered as "clocks" that are based on movement in nature: A sundial shows the time by displaying the position of a shadow on a flat surface.
Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain.
Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.
The Jantar Mantar At Delhi and Jaipur are examples of sundials. Many devices can be used to mark passage of time without respect to reference time (time of day, minutes, etc.) and can be useful for measuring duration or intervals.
Examples of such duration timers are candle clocks, incense clocks and the hourglass.