The calendar.

natural calendar tracks seasonal, biological, and ecological shifts—such as plant flowering, animal migration, and weather changes—to monitor the environment. It acts as a tool for connecting with nature's rhythms, used in citizen science for climate research (e.g., Link: Woodland Trust Nature's Calendar https://naturescalendar.woodlandtrust.org.uk/), as educational tools for students, or as artistic, seasonal journals like the Link: CYCAL Nature's Calendar https://naturescalendar.com/.

Our modern, nearly universal calendar is the Gregorian calendar, introduced by Pope Gregory XIII in October 1582 to correct the Julian calendar’s inaccuracy. It refined the 365.25-day Julian system (introduced by Julius Caesar in 45 BC) by adjusting leap years to better match the 365.2422-day solar year, reducing seasonal drift.

Key aspects of the calendar's evolution include:

Roman Foundations: Originally, Romulus of Rome developed a 10-month calendar.

The Julian Reform (45 BC): Julius Caesar, advised by Sosigenes, established a 12-month, 365-day solar calendar with a leap day every four years.

The Gregorian Correction (1582): The Julian calendar was 10 days out of sync with the seasons. Pope Gregory XIII skipped 10 days (Oct 4, 1582, was followed by Oct 15) and refined the leap year rule: years divisible by 100 are not leap years unless divisible by 400.

Adoption: While Catholic countries adopted it immediately, other nations like Britain took until 1752 to adopt it.

The Gregorian calendar is now the global standard for civil, business, and administrative purposes.

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 10,067 years as of 2025, 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.

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.

For those of you that love the origin of words:

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 calare "to call out", referring to the calling or the announcement that the new moon was just seen. Latin calendarium meant "account book, register", as accounts were settled and debts were collected on the calends of each month. ( it’s all about the money!).

The Latin term was adopted in Old French as calendier and from there into Middle English as calender by the 13th century. The spelling calendar is from Early Modern English.

Other calendars:

Islamic calendar

The ancient Taichu calendar of China

Sub-Saharan African calendars:

Ethiopian Calendar

Nigerian Calendars

Ghana/West African calendar.

Xhosa Calendar- the Xhosa tribe is in South Africa, the most famous Xhosa is South Africa’s first black president Nelson Mandela.

Mesoamerica calendars. (Mayan)

Modern calendars

While the Gregorian calendar is now in worldwide use for secular purposes, various medieval or ancient calendars remain in regional use for religious or social purposes, including the Julian calendar, the Hebrew calendar, the Islamic calendar, various Hindu calendars, the Zoroastrian calendar, etc.

There are also various modern calendars that see limited use, either created for the use of new religious movements or reformed versions of older religious calendars, or calendars introduced by regionalist or nationalist movements.

Javanese calendar (1633)

Jōkyō calendar (1685)

French Republican calendar (1793)

Baháʼí calendar (1873)

Solar Hijri calendar (1925)

Pataphysical calendar (1949)

Indian national calendar (1957)

Discordian calendar (1963)

Juche calendar (1997)