12hour clock

The 12-hour clock was developed over time from mid second milennium BC to the 16th century AD.

The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt: Both an Egyptian sundial for daytime use and an Egyptian water clock for nighttime use were found in the tomb of Pharaoh Amenhotep I. Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each.

The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into four watches. The Romans numbered the morning hours originally in reverse. For example, "3 a.m." or "3 hours ante meridiem" meant "three hours before noon", compared to the modern usage of "three hours into the first 12-hour period of the day".