A Clock is a device used to measure, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.
The 24-hour clock, popularly referred to in the United States and some other countries as military time and the 12-hour time convention which is common in several English-speaking nations and former British colonies, as well as a few other countries.