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. The Yoruba people of West Africa have one of the oldest recorded calendars in human history. It is one of the oldest verified calendar systems in the world used by a continuing culture. Known as Kojoda, the Yoruba calendar dates back over 10,068 years as of 2026, meaning its origin can be traced to approximately 8042 BC. In Victoria, Australia, a Wurdi Youang stone arrangement undergoing research could date back more than 11,000 years. In 2013, archaeologists unearthed ancient evidence of a 10,000-year-old calendar system in Warren Field, Aberdeenshire. 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."Religion in the Etruscan period" in Roman religion in Encyclopædia Britannica
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. 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.
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.
Etymology The term calendars itself is taken from the calends, the term for the first day of the month in the Roman calendar, related to the verb "to call out", referring to the calling or the announcement that the new moon was just seen. Latin 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 and from there into Middle English as by the 13th century. The spelling calendar is from Early Modern English.
Prehistory
Many Upper Paleolithic cave paintings of animals from across western and central Europe are accompanied by sequences of dots and lines. It has been hypothesized that these are tally marks representing lunar months, with the year beginning at the end of winter. A branching "Y" symbol represents mating or birthing month for the associated animal, making each sequence a phenological calendar.
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.
In Victoria, Australia, a Wurdi Youang stone arrangement could date back more than 11,000 years. This estimate is based on the inaccuracy of the calendar, which is consistent with how the Earth's supposed orbit is thought to have changed during that time. The site is found near the world's oldest known site of permanent aquaculture.
A Mesolithic arrangement of twelve pits and an arc found in Warren Field, Aberdeenshire, Scotland, dated to roughly 8,000 BC, has been described as a lunar calendar and was dubbed the "world's oldest known calendar" in 2013.
Antiquity
Ancient Sumerian
The ancient Sumerian calendar, roughly dated to 2100 BC, divided a year into 12 lunar months of 29 or 30 days. Each month began with the sighting of a new moon. Sumerian months had no uniform name throughout Sumer because of the religious diversity. This resulted in scribes and scholars referring to them as "the first month", "the fifth month", etc. To keep the lunar year of 354 days in step with the solar year of 365.242 days an extra month was added periodically, much like a Gregorian leap year. There were no weeks in the Sumerian calendar. Holy days and time off from work were usually celebrated on the first, seventh and fifteenth of each month. In addition to these holy days, there were also feast days which varied from city to city.
Babylonia and Persia
Although the earliest evidence of Iranian calendrical traditions is from the second millennium BC, predating the appearance of the Iranian prophet Zoroaster, the first fully preserved calendar is that of the Achaemenids. Throughout recorded history, Persians have been keen on the idea and importance of having a calendar. They were among the first cultures to use a solar calendar and have long favoured a solar over lunar and lunisolar approaches. The sun has always been a symbol in Iranian culture and is closely related to the folklore regarding Cyrus the Great.
Old Persian calendar Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the solar observation directly and modified for their beliefs. Days were not named. The months had two or three divisions depending on the phase of the moon. Twelve months of 30 days were named for festivals or activities of the pastoral year. A 13th month was added every six years to keep the calendar synchronized with the seasons.
Zoroastrian calendar
The first calendars based on Zoroastrian cosmology appeared in the later Achaemenid period (650 to 330 BC). They evolved over the centuries, but month names changed little until now.
The unified Achaemenid Empire required a distinctive Iranian calendar, and one was devised in Egyptian tradition, with 12 months of 30 days, each dedicated to a yazata (Eyzad), and four divisions resembling the Semitic week. Four days per month were dedicated to Ahura Mazda and seven were named after the six Amesha Spentas. Thirteen days were named after Fire, Water, Sun, Moon, Tiri and Geush Urvan (the soul of all animals), Mithra, Sraosha (Soroush, yazata of prayer), Rashnu (the Judge), Fravashi, Bahram (yazata of victory), Raman (Ramesh meaning peace), and Vata, the divinity of the wind. Three were dedicated to the female divinities, Daena (yazata of religion and personified conscious), Ashi (yazata of fortune) and Arshtat (justice). The remaining four were dedicated to Asman (lord of sky or Heaven), Zam (earth), Manthra Spenta (the Bounteous Sacred Word) and Anaghra Raocha (the 'Endless Light' of paradise).
Modifications by Parthians, Ardashir I, Hormizd I, Yazdgerd III The Parthians (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BC, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the Middle Persian names used by the Sassanians. For example, in Achaemenid times the modern Persian month 'Day' was called Dadvah (Creator), in Parthian it was Datush, and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).
When in April of AD 224 the Parthian dynasty fell and was replaced by the Sasanid, the new king, Ardashir I, abolished the official Babylonian calendar and replaced it with the Zoroastrian. This involved a correction to the places of the gahanbar, which had slipped back in the seasons since they were fixed. These were placed eight months later, as were the epagemonai, the 'Gatha' or 'Gah' days after the ancient Zoroastrian hymns of the same name. Other countries, such as the Armenians and Choresmians, did not accept the change.
The formation of the current Persian calendar in the 11th century
Toghril Beg, the founder of the Seljuq dynasty, had made Esfahan the capital of his domains and his grandson Malik-Shah was the ruler of that city from 1073. An invitation was sent to Khayyam from Malik-Shah and from his vizier Nizam al-Mulk asking Khayyam to go to Esfahan to set up an Observatory there. Other leading astronomers were also brought to the Observatory in Esfahan and for 18 years Khayyam led the scientists and produced work of outstanding quality. During this time Khayyam led work on compiling astronomical tables and he also contributed to calendar reform in 1079.
Cowell quotes the Calcutta Review No 59:
Khayyam measured the length of the year as 365.24219858156 days. Two comments on this result. Firstly it shows an incredible confidence to attempt to give the result to this degree of accuracy. We know now that the length of the year is changing in the sixth decimal place over a person's lifetime. Secondly it is outstandingly accurate. For comparison the length of the year at the end of the 19th century was 365.242196 days, while today it is 365.242190 days.
Classical Greece
The Greeks, as early as the time of Homer, appear to have been familiar with the division of the year into the twelve lunar months but no intercalary month Embolimos or day is then mentioned. Independent of the division of a month into days, it was divided into periods according to the increase and decrease of the moon. Thus, the first day or new moon was called Noumenia. The month in which the year began, as well as the names of the months, differed among the states, and in some parts even no names existed for the months, as they were distinguished only numerically, as the first, second, third, fourth month, etc.