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Methods of timekeeping can be reconstructed for the prehistoric period from at least the Neolithic. The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation. Calendars are explicit schemes used for timekeeping. The first recorded calendars date to the Bronze Age, dependent on the development of writing in the Ancient Near East, the Egyptian and Sumerian calendars. A larger number of calendar systems of the Ancient Near East becomes accessible in the Iron Age, based on the Babylonian calendar. This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew calendar. A great number of Hellenic calendars develop in Classical Greece, and with the Hellenistic period also influence calendars outside of the immediate sphere of Greek influence, giving rise to the various Hindu calendars as well as to the ancient Roman calendar. Calendars in antiquity were usually lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar. Nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan 10-month solar year.〔"Religion in the Etruscan period" in (Roman religion ) in Encyclopedia Britannica 〕 The Roman calendar was reformed by Julius Caesar in 45 BCE. The Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation. The Gregorian calendar was introduced as a refinement of the Julian calendar in 1582 and is today in worldwide use as the ''de facto'' calendar for secular purposes. The term ''calendar'' itself is taken from ''calendae'', the term for the first day of the month in the Roman calendar, related to the verb ''calare'' "to call out", referring to the "calling" of the new moon when it was first seen. Latin ''calendarium'' meant "account book, register" (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as ''calendier'' and from there in Middle English as ''calender'' by the 13th century (the spelling ''calendar'' is early modern). ==Prehistory== 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.〔() 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「history of calendars」の詳細全文を読む スポンサード リンク
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