Where Did the Calendar Come From?

Where did our calendar system come from? The answer involves the sun, moon, and stars—with a dose of Roman Catholicism, colonialism, and globalization.

We live our lives by the calendar. We divide and plan and track our time based on a calendar system that we accept as true and real, but have you ever thought that it has not always been this way, that it doesn’t have to be this way, and in some parts of the world, they use a different calendar system?

The current calendar system used in 168 countries around the world is called the Gregorian calendar. Though we take this common system of reckoning time as a given, it is largely a function of Roman Catholicism, colonialism, and globalization that it has proliferated throughout the world.

Pre-Julian Calendars

Societies throughout the world have generally marked time by following patterns of the sun and moon. Calendar systems developed around the solar cycle (the time it takes the earth to revolve around the sun), the lunar cycle (the 29.5 days from new moon through the eight phases of the moon), and a given society’s festivals and harvest celebrations.

The Gregorian calendar, like many of its precursors, divides time into days based on the earth’s rotation, weeks roughly based on the length of the phases of the moon, months roughly based on the length of the entire moon cycle, and years based on the earth’s revolution around the sun. However, the movements of celestial bodies don’t always conveniently line up with human constructs of time, so we have to compensate by adding extra days here and there to realign with the skies and seasons.

Calendars developed by the Babylonians and Egyptians were the precursors to the modern Gregorian calendar, but they were far from the only ones. For example, the Jewish calendar, the Mayan and Aztec calendars, and the Chinese calendar developed during the Shang dynasty used measurements of the sun, moon, and stars to develop a complex system of chronology to mark time and track agricultural, economic, and ritual events.

The Julian Calendar

In the year 45 BCE, Julius Caesar implemented what was known as the Julian calendar, consisting of a 365-day solar year divided into 12 months and a leap year of 366 days every four years. Julius also moved the start of the new year from March 1 to January 1.

The fall of the Roman Empire led to a gradual shift of the new year to coincide with Christmas Day in many countries. The legal new year was January 1, but by the ninth century, southern European countries had moved their new year to March 25 to coincide with Annunciation Day, and England followed suit several centuries later.

Records often indicate events happening in “the first month” (March), “the second month” (April), and so on, where September (from sept- meaning “seven”) is the seventh month, October (from oct- meaning “eight”) is the eighth month, November (from nov- meaning “nine”) is the ninth month, and December (from deci meaning “ten”) is the tenth month—as it should be.

Now, because the new year has been moved to January, the names of these months do not correspond to their numerical order in the calendar year (e.g., September is the ninth month rather than the seventh month).

The Gregorian Calendar

In the Middle Ages, it became apparent that the Julian calendar had a big flaw: there were too many leap years.

We typically think of a year as 365 days, corresponding to the amount of time it takes the earth to travel around the sun. However, the solar year is actually 365.2422 days, to be exact. The Julian calendar added a leap year every four years, but this formula made the calendar year slightly longer than the solar year over time.

After over 1,500 years, the spring and fall equinoxes fell 10 days early according the calendar in relation to the actual equinoxes determined by the position of the sun. By definition, an equinox is when the sun is directly over the equator, making the day and night of equal length. At the spring equinox, the sun is moving northward, and at the fall equinox, the sun is moving southward. This was a concern to Roman Catholics, who determined the date of Easter with the assumption that the spring equinox was on March 21 each year. The shift in the calendar date relative to the spring equinox meant that Easter, with its reference to Passover and imagery of new life and rebirth, was celebrated earlier and earlier before spring truly started.

So, in 1582, Pope Gregory XVIII proposed the Gregorian calendar. It resolved the leap year issue by implementing a leap year every four years except when the year is divisible by 100—and when the year was divisible by 400, the leap year is added anyway (so 1996 and 2000 were leap years, but not 1900). In addition, the first year the calendar was implemented, 10 days were dropped from the month of October to fix the equinox issue and ensure Easter was celebrated in the spring. Finally, January 1 was officially the date of the new year.

Most Roman Catholic countries quickly adopted the new calendar to correct the dates of observance of religious holidays. Protestants were generally suspicious of the Gregorian calendar and viewed it as an attempt by the pope to slow the spread of Protestantism. Most Orthodox and Protestant countries initially rejected the Gregorian calendar and continued to use the older system. This included England and its colonies in the Americas.

The Change of 1752

After the Gregorian calendar had been in use for about 200 years, there were a total of 11 days’ difference between the two calendar systems, and it was making global communication difficult. Eventually, an Act of Parliament in 1750 put an end to the confusion. The 1752 switch, as it was called, was accomplished in a series of steps that enraged and confused everyone for a bit, but it worked out. Benjamin Franklin even welcomed the new system, writing, “It is pleasant for an old man to be able to go to bed on September 2, and not have to get up until September 14.” (Maybe we’ll make the jump to the metric system at some point too—who knows?)

Due to the discrepancies in the Julian and Gregorian calendar systems, both in use from 1582 to 1752, a double-dating system developed that included both the “old style” and “new style” date. You might see dates like February 8, 1686/7 in civil records from England or the American colonies. Historians today typically preserve the old style date in transcriptions. But directly after the change, some people converted significant dates to the new style. For example, George Washington was born on February 11, 1731 according to the Julian calendar, but he later used  February 22, 1732 as his birth date in line with the Gregorian calendar.

The Spread of the Gregorian Calendar

Over the course of centuries, Catholic, then Protestant, then Orthodox European countries adopted the Gregorian calendar system. By the early twentieth century, Western nations were largely using the same calendar, and so were the areas formerly or currently colonized by them. Many other nations in the world eventually followed to allow ease of communication and business as the world became more interconnected.

Other Calendar Systems

Some countries that have adopted the Gregorian calendar for civil and administrative purposes have also retained another calendar system alongside it for religious or cultural purposes. For example, Israel uses the Hebrew calendar for religious purposes and the Gregorian calendar for civil purposes. China officially uses the Gregorian calendar, but the traditional Chinese lunar-solar calendar is used to determine cultural holidays, including Chinese New Year (Spring Festival) and Shangyuan (Lantern Festival). It is also used to determine fortunate days for weddings and other events.

Map of civil calendars used around the world.
Image by Weaveravel, CC BY-SA 4.0 via Wikimedia Commons.

Other countries do not use the Gregorian system at all and use another calendar system for all purposes. Iran and Afghanistan, for example, use the Solar Hijri calendar, an ancient Iranian calendar that begins on the spring equinox. Because it uses astronomical calculations to determine the equinox, it is considered the most accurate solar calendar currently in use. This day is called Nowruz (“New Day”) and is one of the biggest celebrations in countries that use the Solar Hijri calendar and countries with a strong Iranian/Persian influence. The names of the 12 months come from the Zoroastrian calendar and correspond to the 12 zodiac signs. The first six months have 31 days, the next five months have 30 days, and the last month has 29 days unless it’s a leap year, in which case it has 30. Its epoch (the year from which the calendar year is determined) is 500 BCE.

Ethiopia has a rather interesting calendar system as well. The Ethiopian or Ge’ez calendar is derived from the Coptic Egyptian calendar and was first used by the Ethiopian Orthodox Tewahedo Church. It is used today as the liturgical calendar in various Ethiopian churches as well as the administrative calendar for the country. The Ethiopian calendar usually begins on the the equivalent of September 11 on the Gregorian calendar. Its epoch is held to be the Annunciation of Christ, but it uses an alternative calculation—placing it about seven years behind the Gregorian calendar system. This current era is called the Incarnation Era. The Ethiopian calendar has 13 months—the first 12 are 30 days each, and the 13th month gets five or six days to align with the solar year. Due to all the differences between the calendar systems, on January 16, 2023, on the Gregorian calendar, it is August 8, 2015 on the Ethiopian calendar.

As one last example, the Thai solar calendar is used as the administrative calendar of Thailand alongside the Thai lunar calendar for cultural and religious events. The Thai solar calendar is essentially a renumbering of the Gregorian calendar and was adopted in 1888 CE. The epoch is the passing away of the Buddha, called the Parinibbāna, corresponding to the Gregorian year 543 BCE. The current era is known as the Buddhist Era, which is shared by a set of Buddhist lunar-solar calendars used in southeast Asia. These calendars ultimately derive from the Hindu calendar, and the names of the months of the Thai solar calendar are derived from the Hindu names for the signs of the zodiac. The traditional new year is celebrated in April, when the sun crosses the constellation of Aries—the first astrological sign in the zodiac. However, in 1941, the prime minister declared that January 1 would be the start of the new year. Another interesting part of the Thai calendar system is that each day of the week corresponds to a color, a celestial body, and a deity. For example, Monday is associated with the color yellow, the moon, and Chandra, the Hindu god of the moon. Wednesday is divided into day and night, each with its own correlations.

Reckoning with Time

So, the fact is that our reckoning of time is not unchanging nor universal. Though it makes perfectly good sense to those who use it, the Gregorian calendar has been shaped by geography and climate, by religious holidays and agricultural seasons, by the ordering of time in linear fashion rather than in a cyclical manner, as some societies do. It has spread across the globe for reasons both good and bad, but the fact is that this system in turn shapes the way we think about time and the way we order our lives.

But—we’re still slightly short of the full solar year. Someday, it just might catch up to us and we’ll all have to get on board with another calendar realignment.  


Jennie Cohen, “6 Things You May Not Know About the Gregorian Calendar.” History.com, August 22, 2018. https://www.history.com/news/6-things-you-may-not-know-about-the-gregorian-calendar.

Encyclopaedia Brittanica, “Calendar.”

Connecticut State Library, “The 1752 Calendar Change,” https://libguides.ctstatelibrary.org/hg/colonialresearch/calendar.

Amber Pariona, “What Is the Gregorian Calendar?” World Atlas, April 25, 2017. https://www.worldatlas.com/articles/what-is-the-gregorian-calendar.html.

Sarah W. Wanza, “Countries That Use Their Own Calendar.” World Atlas, May 29, 2018. https://www.worldatlas.com/articles/countries-that-use-their-own-calendar.html

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