History of calendars

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The history of calendars covers practices with ancient roots as people created and used various methods to keep track of days and larger divisions of time. Calendars commonly serve both cultural and practical purposes and are often connected to astronomy and agriculture.

Archeologists have reconstructed methods of timekeeping that go back to prehistoric times at least as old as 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 historically attested and formulized calendars date to the Bronze Age, dependent on the development of writing in the ancient Near East. In 2000 AD, Victoria, Australia, a Wurdi Youang stone arrangement could date back more than 11,000 years.^[1] In 2013, archaeologists unearthed ancient evidence of a 10,000-year-old calendar system in Warren Field, Aberdeenshire.^[2] This calendar is the next earliest, or "the first Scottish calendar". The Sumerian calendar was the next earliest, followed by the Egyptian, Assyrian and Elamite calendars.

The Vikram Samvat has been used by Hindus and Sikhs. One of several regional Hindu calendars in use on the Indian subcontinent, it is based on twelve synodic lunar months and 365 solar days. The lunar year begins with the new moon of the month of Chaitra. This day, known as Chaitra Sukhladi, is a restricted (optional) holiday in India. A number of ancient and medieval inscriptions used the Vikram Samvat. Although it was purportedly named after the legendary king Vikramaditya Samvatsara (‘Samvat’ in short), ‘Samvat’ is a Sanskrit term for ‘year’. Emperor Vikramaditya of Ujjain started Vikram Samvat in 57 BC and it is believed that this calendar follows his victory over the Saka in 56 B.C.

A larger number of calendar systems of the ancient East appear in the Iron Age archaeological record, based on the Assyrian and Babylonian calendars. This includes the calendar of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew 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.^[3]

The Roman calendar was reformed by Julius Caesar in 45 BC. 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.

Sub-Saharan African calendars can vary in days and weeks depending on the kingdom or tribe that created it.

In the 11th century in Persia, a calendar reform led by Khayyam was announced in 1079, when the length of the year was measured as 365.24219858156 days.^[4] Given that the length of the year is changing in the sixth decimal place over a person's lifetime, this is outstandingly accurate. For comparison the length of the year at the end of the 19th century was 365.242196 days, while at the end of the 20th century it was 365.242190 days.^[4][5]

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.