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A number of prehistoric structures have been proposed as having had the purpose of timekeeping (typically keeping track of the course of the solar year). This includes many megalithic structures, and reconstructed arrangements going back far into the Neolithic period.
A mesolithic arrangement of twelve pits and an arc found in Warren Field, Aberdeenshire, Scotland, dated to roughly 10,000 years ago, has been described as a lunar calendar and dubbed the "world's oldest known calendar" in 2013
Most of the oldest calendars were lunar calendars, based on the time interval from one new moon to the next—a so-called lunation. But even in a warm climate there are annual events that pay no attention to the phases of the Moon. In some areas it was a rainy season; in Egypt it was the annual flooding of the Nile River. The calendar had to account for these yearly events as well.
- History of the Lunar Calendar
- History of the Egytpian Calendar
- History of the Roman (Julian) Calendar
- History of the Gregorian Calendar
- Adoption of the Gregorian Calendar