Ancient Greece: The Water Clock (Clepsydra) Of Ktesibios

Water clocks were placed on public places in Ancient Greece. Even more often you could see them in court, to limit the speech time.
Water fills a canister. A float is lifted. A figure is mounted on its top, which points to a cylinder-shaped board. On the board are 24 squares, which stand for hours.
When the canister is full (24 hours have passed), the water flows through the pipe. The out flown water powers a gear, which rotates the board. The further the board rotates, the more days pass.

This simple piece of mechanics, adjusted rightly, could be quite precise.