A new year has just started, and 2024 is going to be special, as it is a leap year in the Gregorian calendar, the most used calendar around the world. To understand why this calendar is the de facto international standard, and why 2024 is a leap year, we need to go through the history and evolution of this calendar, and then we will take a look at the other calendars used in different countries.
Before starting, let’s define a few useful terms. A solar calendar is a calendar based solely on the position of the Sun, with a year corresponding to a full orbit of the Earth around the Sun. Meanwhile, a lunar calendar is based on the monthly cycle of the Moon orbiting the Earth. Finally, a lunisolar calendar is a combination of the two, usually with lunar months and an occasional intercalation to align with the solar year. This is the time that the Sun takes to return to the same position in the sky (also called tropical year), and corresponds to 365.24219 days.
Humans have been keeping track of time since prehistory, and various megalithic structures built around the world since at least the Neolithic were most likely used as calendars, marking the position of the Sun and other astronomical objects.
In ancient Mesopotamia, the Sumerians developed a lunisolar calendar with twelve lunar months and periodically added an extra month to align with the solar year. Originally created around 2100 BCE, this calendar was later used and adapted by the Babylonians and other civilizations of the Near East. Meanwhile, the Egyptians used a solar calendar with twelve months of 30 days each, and five days added at the end for a total of 365 days.
Various lunisolar calendars were developed in ancient Greece, while the Romans initially used a lunar calendar with ten months of 30 or 31 days, for a total of 304 days, while the winter was unorganized. Two months were later added at the start of the year, January and February. However, the original names of the months were left unchanged, and are still used today, hence why we still have September, meaning “seventh month”, while it is actually the ninth month, and so on for October, November, and December.
This calendar was later changed into a lunisolar one similar to those used by the Greeks. The Roman calendar during the Republican age had twelve months of 29 or 31 days, with only February having 28. Every two years an intercalary month of 27 or 28 days was added, while February was shortened to 23 days. So, four consecutive years had 355, 377, 355, and 378 days, but this cycle only repeated for sixteen years, followed by seven years in which the intercalation always had 27 days, and a final year with no intercalation. This resulted in a 24-year cycle in which the year had a mean of 365.25 days, close to the length of the tropical year.
Reconstruction of the Fasti Antiates Maiores, a Roman calendar made between 84 BCE and 55 BCE and found in Anzio, Italy. This reconstruction is in the Cesaraugusta Theater Museum in Zaragoza, Spain. The calendar shows thirteen months including an intercalary month, while July and August are still called Quintilis and Sextilis, before being renamed in honor of Julius Caesar and Augustus (Bauglir, Wikimedia Commons, CC BY-SA 4.0).
In 46 BCE Julius Ceasar introduced a new simpler system, which became known as Julian calendar. This solar calendar is very similar to the one we currently use, with twelve months of 30 or 31 days and only February having 28, for a total of 365 days. Every four years a day is added to February so that the average year is 365.25 days long. This calendar was used in the Roman Empire, and became the standard in Europe for centuries. However, since the average year is slightly longer than the tropical year, the calendar gains a day every 129 years. This caused the day of the equinoxes and solstices to slowly drift, influencing the date of Easter, which depends on the spring equinox.
For this reason, in 1582 a new calendar was introduced by Pope Gregory XIII, now known as Gregorian calendar. This system mostly keeps the standard Julian calendar, only changing which years have 366 days. In the Gregorian calendar, every year divisible by four is a leap year, except those that are divisible by 100, but the years divisible by 400 are leap years. This means that 2024, 2028, 2032 etc. are all leap years, 1700, 1800, and 1900 were not leap years, but 1600 and 2000 were. The average year in the Gregorian calendar is 365.2425 days long, still slightly longer than the tropical year. With this approximation, the error is about one day every 3000 years.
The Gregorian calendar was quickly adopted by Catholic countries, but Protestant and Orthodox churches initially opposed the reform. This calendar was eventually accepted by the other European countries over the following centuries, most notably by the British Empire in 1752, which spread this system around the world. Countries in Eastern Europe and Asia adopted the Gregorian calendar between the late 19th and early 20th century. Now most countries use this system as their civil calendar, while some keep their national or traditional calendars mostly for religious purposes and to determine holidays, and often alongside the Gregorian calendar. A revised Julian calendar remains in use by some Orthodox churches.
One of the first calendars printed after the Gregorian reform, the Lunario Novo, Secondo la Nuova Riforma della Correttione del l’Anno Riformato da N.S. Gregorio XIII, printed in Rome in 1582. Ten days were skipped to account for the extra leap years added by the Julian calendar, so here 4 October is followed by 15 October.
Other then the Gregorian calendar, the most widely used systems are the Chinese, Islamic, and Hindu calendars. The Chinese calendar was initially developed during the Spring and Autumn period, between the 8th and 5th century BCE. Different states and dynasties used various versions of solar or lunisolar calendars. One of these is the Wuxing (or five phases) solar calendar, which divides the year into five phases of 73 days, each starting with a day dedicated to one of the five elements (fire, water, wood, metal, earth), followed by six weeks of twelve days. A month has three weeks, so a year has ten months of 36 days each and five extra days, for a total of 365 days.
Other lunisolar calendars with twelve or thirteen lunar months were later adopted and, after various reforms, the length of a year was fixed as 365.2425 days, the same as the Gregorian calendar. The current Chinese calendar has twelve lunar months of 29 or 30 days for a total of between 353 and 355 days. Intercalary months are periodically added to align with the solar year, and leap years have between 383 and 385 days. A year is a leap year if there are thirteen new moons from the start of the 11th month in the first year to the start of the 11th month in the following year. This calendar is now only used to determine the days of holidays and festivals, while the Gregorian calendar is used for official matters in China.
Meanwhile, Taiwan uses the Minguo calendar, which is basically the same as the Gregorian calendar but counting the years since the foundation of the Republic of China in 1912. Curiously, North Korea has the same system, as they use the Juche calendar which counts the years starting with the birth of Kim Il Sung in 1912. Similarly, the Japanese calendar is the same as the Gregorian calendar but counting the years since the start of the reign of the current emperor.
A page of a North Korean calendar from 2010 also showing the year as Juche 99 (Roman Harak, Flickr, CC BY-SA 2.0).
Another calendar with a long history is the Hindu calendar, which was originally introduced in the Vedic period (1500 BCE-500 BCE). This is a lunisolar calendar which, like other similar systems, has twelve lunar months with a total of 354 days. An extra month is periodically added to line up with the solar year, with the intercalary months timed on a 19-year cycle. This calendar is used for religious and traditional purposes, but India officially uses the Indian national calendar, which counts the years in the Shaka era, setting the year zero on 78 CE, probably referring to the ascension of Indo-Scythian king Chashtana.
Other similar lunisolar calendars are used in India, such as the Vikram Samvat, which is 56 or 57 years ahead of the Gregorian calendar, while Nepal uses a system called Nepal Sambat, which counts the years starting on 20 October 879. According to a local legend, on that day a merchant called Sankhadhar Sakhwa paid all the debts of the Nepali people.
Various versions of the Buddhist calendar are used in South and Southeast Asia, and are based on the Hindu calendar. These systems are lunisolar calendars with alternating months of 29 and 30 days, and periodically add intercalary days and months. The numbering of the years begins on the day on which Buddha attained parinirvana (nirvana after death), which is traditionally said to have happened in 544 BCE. As this calendar also has a year zero, the Buddhist Era is 543 years ahead of the Gregorian calendar. Thailand officially uses a version of the Gregorian calendar which just adds 543 to the current Gregorian year.
Another widely used system is the Islamic calendar. Initially, pre-Islamic Arabic peoples used lunar or lunisolar calendars which distinguished between permitted (ḥalāl) and forbidden (ḥarām) months. During the forbidden months, fighting was prohibited. At some point, the concept of nasi’ (postponement) was introduced. Various interpretations of this concept have been proposed, suggesting that religious events were postponed, maybe in correlation to intercalary months. Nevertheless, the nasi’ was prohibited in the Quran, and the Islamic calendar has since been a purely lunar calendar.
The twelve months of the Islamic calendar have 29 or 30 days, resulting in a year of 354 or 355 days. As there is no correction to align with the solar year, the Islamic calendar shifts by about eleven days each year as compared to the Gregorian calendar. The years are counted from the Hijrah, the journey of Muhammad and his followers from Mecca to Medina in 622.
The Islamic calendar is used to determine the dates of Islamic holidays, while most countries with a Muslim majority still use the Gregorian calendar as their civil calendar. Bangladesh uses a calendar based on a system introduced by the Mughal Emperor Akbar in 1584, that was influenced by the Islamic and Hindu calendars, counting the years starting in 594, regarded as the date of the rise of Shashanka, the first king of an independent and unified Bengali kingdom.
A Tunisian calendar from 1999, showing both the Gregorian and Islamic dates (Imprimerie Dar el Founoun, Wikimedia Commons, CC BY-SA 3.0).
Iran uses a solar calendar that counts the years from the Hijrah, called the Solar Hijri calendar, but is different from the Islamic system. The Solar Hijri calendar has six months of 31 days, followed by five months of 30 days, and a final month with 29 days, or 30 days in leap years. As the Islamic calendar is purely lunar and shorter than the solar year, the years in the Iranian and Islamic calendars slowly drift apart, and currently have a difference of 43 years. Afghanistan also used to have the Solar Hijri calendar, but since the return of the Taliban rule in the country, the Islamic calendar has been officially adopted.
The Hebrew calendar is the official calendar in Israel, and it is used to determine the dates of Jewish festivals. This is a lunisolar system with twelve months of 29 or 30 days and an intercalary month added every two or three years following a 19-year cycle. The years are counted since the creation of the world based on calculations made from biblical accounts, a system called Anno Mundi (“year of the world” in Latin). According to Jewish tradition, this happened in 3761 BCE, so the current Hebrew year (between 15 September 2023 and 2 October 2024) is 5784.
Finally, Ethiopia uses a solar calendar which, similarly to the Julian calendar, adds a leap day every four years without exception. Every year has twelve months of 30 days, with five or six extra days, and the year starting on 11 or 12 September of the Gregorian calendar. The Ethiopian calendar is seven or eight years behind the Gregorian calendar due to a different calculation of the date of the birth of Jesus.
Map of civil calendars used around the world (Weaveravel, Wikimedia Commons, CC BY-SA 4.0).
