The Gregorian Calendar

The Gregorian calendar is today's internationally accepted civil calendar and is also known as the Western or Christian calendar.

12 Irregular Months

The Gregorian Calendar is the most widely used calendar in the world today. It is the calendar used in the international standard for Representation of dates and times: ISO 8601:2004.

It is a solar calendar based on a 365-day common year divided into 12 months of irregular lengths. 11 of the months have either 30 or 31 days, while the second month, February, has only 28 days during the common year. However, nearly every four years is a leap year, when one extra – or intercalary – day, is added on 29 February, making the leap year in the Gregorian calendar 366 days long.

The days of the year in the Gregorian calendar are divided into 7-day weeks, and the weeks are numbered 1 to 52 or 53. The international standard is to start the week on Monday. However, several countries, including the US and Canada, count Sunday as the first day of the week.

Replaced Julian Calendar

The Gregorian calendar's predecessor, the Julian Calendar, was replaced because it was too inaccurate. It did not properly reflect the actual time it takes the Earth to orbit once around the Sun, known as a tropical year.

Realigned With the Sun

The Julian calendar's formula to calculate leap years produced a leap year every four years. This is too often, and eventually, the Julian calendar was several days out of sync with the fixed dates for astronomical events like equinoxes and solstices.

The introduction of the Gregorian calendar allowed for the realignment with events like the vernal equinox and winter solstice.

New Leap Year Formula

The Gregorian calendar was first adopted in Italy, Poland, Portugal, and Spain in 1582, and included the following changes:

  • New formula for calculating leap years:

1.      The year is evenly divisible by 4;

2.      If the year can be evenly divided by 100, it is NOT a leap year, unless;

3.      The year is also evenly divisible by 400: Then it is a leap year.

  • 10 days were dropped in October 1582
  • New rules for calculating Easter dates

Protestant Countries Were Sceptical

Catholic countries, such as Spain, Portugal, and Italy, quickly adopted Pope Gregory’s calendar reforms for their civil affairs. In Europe's Protestant countries, however, people feared that the new calendar was an attempt by the Catholic Church to silence their movement. It took almost 200 years before England and the colonies switched over when an act of Parliament introduced the new calendar, advancing the date from September 2 to September 14, 1752.

Benjamin Franklin famously wrote about the switch in his almanac: "...And what an indulgence is here, for those who love their pillow to lie down in Peace on the second of this month and not perhaps awake till the morning of the fourteenth." (Quoted by Cowan, 29; Irwin, 98)

Orthodox countries followed the Julian calendar even longer, and their national churches have still not adopted Pope Gregory XIII’s calendar.

Proleptic Gregorian Calendar

If you extend the Gregorian calendar backward to dates before it was officially introduced in 1582, it is called the proleptic Gregorian calendar. The standard ISO 8601:2004 requires dates before 1582 to be expressed in this format (clause The Gregorian calendar)

Is Any Calendar Perfect?

The more advanced leap year formula makes the Gregorian calendar far more accurate than the Julian. However, it is not perfect either. Compared to the tropical year, it is off by one day every 3236 years.

Who Designed the Calendar?

Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Luigi Lilio (also known as Aloysius Lilius), who was an Italian doctor, astronomer, and philosopher. He was born around 1510 and died in 1576, six years before his calendar was officially introduced.

The Julian Calendar

The Julian calendar was introduced by Julius Caesar in 45 BCE and replaced the Roman calendar.

Replaced Lunar Calendar

The Julian calendar's predecessor, the Roman calendar, was a very complicated lunar calendar, based on the moon phases. It required a group of people to decide when days should be added or removed in order to keep the calendar in sync with the astronomical seasons, marked by equinoxes and solstices.

First Solar Calendar

In order to create a more standardized calendar, Julius Caesar consulted an Alexandrian astronomer named Sosigenes and created a more regulated civil calendar, a solar calendar based entirely on Earth's revolutions around the Sun, also called a tropical year. It takes our planet on average, approximately 365 days, 5 hours, 48 minutes and 45 seconds (365.242189 days) to complete one full orbit around the Sun.

Introducing Leap Years

A common year in the Julian calendar has 365 days divided into 12 months.

In the Julian calendar, every four years is a leap year, with a leap day added to the month of February.

At the time, February was the last month of the year, and Leap Day was February 24.

However, leap years were not observed in the first years after the reform due to a counting error. In the first years of the Julian calendar’s existence – until 12 Common Era (CE) – every third year was a leap year due to a calculation error.

Too Many Leap Days

The Julian calendar's formula to calculate leap years produced a leap year every four years. This is way too often, and eventually the Julian calendar and important religious holidays, like Easter were several days out of sync with the fixed dates for astronomical events like equinoxes and solstices.

Realigned With the Sun

The introduction of the Gregorian calendar allowed for the realignment with events like the Vernal equinox and Winter solstice.

The solution to this error was to replace the Julian calendar with the Gregorian calendar in 1582.

The Julian Calendar in Modern Society

Although the Gregorian calendar has become the international civil calendar, the Julian calendar was still used by some countries into the early 1900s. Some Orthodox churches still use it today to calculate the dates of moveable feasts, such as the Orthodox Church in Russia. Others who still use the Julian calendar include the Berber people of North Africa and on Mount Athos.

The calendar was used throughout the Roman Empire and by various Christian churches.

The Julian Period for Astronomers

The Julian period or the Julian Day system provides astronomers with a single system of dates that could be used when working with different calendars to align different historical chronologies. It assigns a Julian Day (JD) to every year without having to worry about B.C.E or C.E. It was invented by French Scholar Joseph Justus Scaliger in 1583, who proposed that the Julian Period starts at noon on January 1, 4713 B.C.E. (Julian calendar) and lasts for 7980 years. This was determined because it is a time period long enough to include all of recorded history and includes some time in the future that would incorporate the three important calendrical cycles, the Golden Number Cycle, the Solar Cycle, and the Roman Indiction.

The Golden Number Cycle is a cycle of 19 years, while the Solar Cycle is a cycle of 28 years and the Roman Indiction repeats every 15 years. Thus the Julian Period is calculated to be 7980 years long or 2,914,695 days because 19*28*15 = 7980.

Change From Julian to Gregorian Calendar

The Gregorian calendar was first introduced in 1582, but it took more than 300 years for all the different countries to change from the Julian Calendar.

The Gregorian Calendar, also known as the Western or Christian Calendar, is the most widely used calendar in the world today. Its predecessor, the Julian Calendar, was replaced because it did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.

Too Many Leap Years

The reason the Julian Calendar had to be replaced was the formula it used to calculate leap years. The Julian formula produced a leap year every four years, which is too many. The Gregorian Calendar uses a much more accurate rule for calculating leap years.

Skipped Several Days

To get the calendar back in sync with astronomical events like the vernal equinox or the winter solstice, a number of days were dropped.

The papal bull issued by Pope Gregory XIII in 1582, decreed that 10 days be dropped when switching to the Gregorian Calendar. However, the later the switch occurred, the more days had to be omitted. (See table below).

This created short months with only 18 days and odd dates like February 30 during the year of the changeover.

In North America, the month of September 1752 was exceptionally short, skipping 11 days.

Switch Took More Than 300 Years

The Gregorian Calendar was first introduced in 1582 in some European countries (*). However, many countries used the Julian Calendar much longer. Turkey was the last country to officially switch to the new system on January 1, 1927.


Year of Switch                   Country                                                                       Days Removed

1582                                  France (most areas), Italy, Poland, Portugal, Spain              10 days

1583                                  Austria, Germany (Catholic states),                                    10 days

1587                                  Hungary,                                                                         10 days

1610                                  Germany (Prussia),                                                           10 days

1752                                  United States (most areas), Canada (most areas),

                  United Kingdom (and colonies)                                         11 days

1872/1873                          Japan,                                                                           12 days

1916                                  Bulgaria,                                                                        13 days

1918                                  Estonia, Russia,                                                              13 days

1923                                  Greece,                                                                          13 days

1926/1927                         Turkey,                                                                          13 days

* Note: The list includes only a small selection of countries. In some cases, it shows a simplified version of events. Each country is listed by its current name although its official name may have changed since the calendar reform.

The delay in switching meant that different countries not only followed different calendars for a number of years but also had different rules to calculate whether a year was a leap year.

This explains why the years 1700, 1800, and 1900 were leap years in countries still using the Julian calendar (e.g. Greece), while in countries that had adopted the Gregorian calendar (e.g. Germany), these years were common years.

Double Leap Year

Sweden and Finland had a "double" leap year in 1712. Two days were added to February, creating February 30, 1712. This was done because the Leap Year in 1700 was dropped and Sweden's calendar was not synchronized with any other calendar. By adding an extra day in 1712, they were back on the Julian calendar. Both countries introduced the Gregorian calendar in 1753.

Japan replaced its lunisolar calendar with the Gregorian calendar in January 1873, but decided to use the numbered months it had originally used rather than the European names.

The Republic of China originally adopted the Gregorian calendar in January 1912, but it wasn’t used in China due to warlords using different calendars. However, the Nationalist Government formally decreed the adoption of the Gregorian calendar in China in January 1929.

13 Days Behind Today

Currently (1901–2099), the Julian calendar is 13 days behind the Gregorian calendar.

Today's Gregorian calendar uses more accurate leap year formula, making it far more accurate than the Julian. However, it is not perfect either. Compared to the tropical year, it is off by one day every 3236 years.

The Revised Julian Calendar

This Revised Julian calendar uses even more complex rules to determine when to add a leap day. With an error of only about two seconds per year (or one day in 31,250 years), it is roughly 10 times more accurate than today's Gregorian calendar and one of the most accurate calendar systems ever devised. However, it is not used by any country, only by certain orthodox churches.

The Roman Calendar

The Roman calendar is the ancestor of our modern calendar. Some of its features are still in use today.

What Is the Roman Calendar?

The Roman calendar is the time reckoning system used in ancient Rome. However, because the calendar was reformed and adjusted countless times over the centuries, the term essentially denotes a series of evolving calendar systems, whose structures are partly unknown and vary quite a bit.

Here, we focus mainly on the calendar used in the Roman Republic (509-27 BCE). Also known as the Republican calendar, it is the earliest calendar system from Rome for which we have historical evidence. It was used until 45 BCE, when it was replaced by the Julian calendar.

Based on Ancient Lunar Calendars

The Republican calendar was derived from a line of older calendar systems whose exact design is largely unknown. It is believed that the original Roman calendar was a lunar calendar that followed the phases of the Moon. This basic structure was preserved through the centuries, which is the reason why we use months today.

Only 10 Months at First

According to tradition, Romulus, the legendary first king of Rome, oversaw an overhaul of the Roman calendar system around 738 BCE. The resulting calendar, whose structure borrowed heavily from the ancient Greek calendar system, had only 10 months, with March (Martius) being the first month of the year. The winter season was not assigned to any month, so the year only lasted 304 days, with 61 days unaccounted for in the winter.

Republican Calendar Adds January and February

Months in the Republican Calendar

(Common Year)

Month Names Number of Days

Ianuarius         29

Februarius       28

Martius            31

Aprilis             29

Maius              31

Iunius             29

Quintilis          31

Sextilis            29

September      29

October          31

November       29

December       29

Following another calendar reform, which later Roman writers attributed to Romulus' successor, Numa Pompilius, the Republican calendar was instituted. To account for the days of winter between the years, two additional months were introduced: Ianuarius and Februarius.

This meant that some of the month names no longer agreed with their position in the calendar. For example, September means “the 7th month,” but it was now the 9th month of the year—an inconsistency that was preserved and is still part of the Gregorian calendar we use today.

A common year was now divided into 12 months of different lengths: 4 “full” months with 31 days, 7 “hollow” months with 29 days, and 1 month with 28 days.

The Leap Month, Mercedonius

The Republican calendar year lasted for 355 days, which is about 10 days shorter than a tropical year, the time it takes Earth to revolve around the Sun. To keep the calendar in sync with the seasons, a leap month called Mercedonius or Intercalaris was added in some years—normally every two to three years.

By custom, the insertion of the leap month was initiated by the pontifex maximus, the high priest of the College of Pontiffs in ancient Rome. However, this system was vulnerable to abuse. Since the Roman calendar year defined the term of office of elected officials, a pontifex maximus was able to control the length of his term simply by adding a leap month.

When Julius Caesar became pontifex maximus, he ordered a calendar reform which eliminated leap months and resulted in the implementation of the Julian calendar in 45 BCE, the direct predecessor of today's Gregorian calendar.

Calends, Nones, and Ides

The Roman calendar highlighted a number of days in each month:

  • Calends (Kalendae) were the first days of each month. The name is derived from the Greek word καλειν, to announce, which may initially have been used in the ancient lunar calendar to “announce” the day of the New Moon (or the first sliver of the Waxing Crescent Moon).
  • Ides (Idus) occurred one day before the middle of each month. Depending on the month's length, it fell on the 13th or 15th day. In the lunar calendar, the Ides marked the day of the Full Moon.
  • Nones (Nonae) fell on the 7th day of 31-day months and on the 5th day of 29-day months, marking the day of the First Quarter Moon.

These markers were used to number the days in each month, counting backward from the upcoming Calends, Ides, or Nones. The count always included the day of the marker. For example, the 11th day of Martius would be known as “Five Ides” to the Romans because it is the fifth day before the Ides of Martius, which fell on the 15th day.

Common Era (CE) and Before Common Era (BCE)

The letters CE or BCE in conjunction with a year mean after or before year 1.

  • CE is an abbreviation for Common Era.
  • BCE is short for Before Common Era.

The Common Era begins with year 1 in the Gregorian calendar.

Instead of AD and BC

CE and BCE are used in exactly the same way as the traditional abbreviations AD and BC.

  • AD is short for Anno Domini,
  • Latin for year of the Lord.
  • BC is an abbreviation of Before Christ.

Because AD and BC hold religious (Christian) connotations, many prefer to use the more modern and neutral CE and BCE to indicate if a year is before or after year 1.

According to the international standard for calendar dates, ISO 8601, both systems are acceptable.

Both in Use for Centuries

The Anno Domini year–numbering system was introduced by a Christian monk named Dionysius Exiguus in the 6th century. The year count starts with year 1 in the Gregorian calendar. This is supposed to be the birth year of Jesus, although modern historians often conclude that he was born around 4 years earlier.

The expression Common Era is also no new invention, it has been in use for several hundred years. In English, it is found in writings as early as 1708. In Latin, the term "vulgaris aerae" (English, Vulgar Era) was used interchangeably with "Christian Era" as far back as in the 1600s.

More and More Use CE/BCE

What is relatively new is that more and more countries and their educational institutions have officially replaced the traditional abbreviations AD/BC with CE/BCE.

England and Wales introduced the CE/BCE system into the official school curriculum in 2002, and Australia followed in 2011. More and more textbooks in the United States also use CE/BCE, as well as history tests issued by the US College Board.

Avoid Confusion

A year listed without any letters is always Common Era, starting from year 1.

Adding CE or BCE after a year is only necessary if there is room for misunderstanding, e.g. in texts where years both before and after year 1 are mentioned.

For instance, Pompeii, Italy was founded around 600–700 BCE and was destroyed when Mount Vesuvius erupted in 79 CE.

When Did the 21st Century Start?

On January 1, 2000, humankind celebrated the beginning of the new millennium—which was one year too early.

The 21st Century Started in 2001

In 1999, the world was preparing for the New Year's party of a lifetime. The year number in the Gregorian calendar was about to tick over to 2000, supposedly ushering in not only the 21st century but also the 3rd millennium CE.

However, the party was held one year too early—it should have been on January 1, 2001.

Year Zero

It all boils down to the question: was there a year 0? Let's first assume that year BCE 0 existed. This would mean that:

  • 1 full year would have passed at the end of year 0 since the beginning of the year count;
  • 2 years would have passed at the end of year 1;
  • and so on...

This means that 2000 years, two full millennia, would have passed at the end of year 1999. In other words, the 3rd millennium would have started on New Year's Day 2000.

The only problem with this theory is that year 0 did not exist, as historians, calendar experts,, and other killjoys kept pointing out in the lead-up to the big party in year 2000.

So, how do we know there was no year zero?

Anno Domini

Anno domini, the year numbering system (calendar era) we use today, was devised by a 6th-century monk named Dionysius Exiguus, who lived in an area now part of Romania and Bulgaria. Dionysius used Roman numerals to number the years “since the incarnation of our Lord Jesus Christ”, as he put it in his writings—and there is no Roman numeral for the number zero.

It is worth pointing out that Dionysius did use the Latin number words nulla and nihil, both meaning “nothing,” in his calculations of the date of Easter. However, he used these words to imply the absence of a number, rather than the number zero itself. In fact, it is believed that the concept of the number zero, as it is used today, did not exist in Europe until the 13th century.

Year 1 BCE Was Followed by Year CE 1

This means that year AD 1 directly followed year 1 BC, without the year count ever reaching zero. In other words, the first year of the anno domini era was year 1, not year 0. As a consequence,

  • 1 full year had passed at the end of year 1;
  • 2 years had passed at the end of year 2;
  • and so on...

So, at the end of year 1999, as people were celebrating the new millennium, only 1999 full years had passed since the beginning of the calendar era—which is one year short of two full millennia.

Round Number Bias

Of course, the big fuss over the year 2000, or Y2K, was understandable from a psychological point of view. The human brain is predisposed to highlight “big numbers”—a tendency psychologists call the round number bias. It causes us to throw extra-glamorous parties on our 20th, 30th, or 40th birthdays and to celebrate milestones like the 1000th like on Facebook or the 100th victory of our favorite sports team.

So, in celebrating the beginning of the year 2000 as the event of a lifetime, we collectively succumbed to the appeal of the big, round year number—a classic case of round number bias.

What Did We Even Celebrate?

Even people celebrating the beginning of the new millennium on the correct date must contend with the fact that, in astronomical terms, there was nothing special about this particular event.

A year on Earth is defined as the time it takes Earth to complete an orbit around the Sun. This is called a solar or tropical year. Solar calendar systems, such as the western calendar, are designed to reflect that time span and the seasons it encompasses as precisely as possible, and they do so with varying levels of accuracy.

Year numbers, on the other hand, are just a means of counting our planet's revolutions. While this is undoubtedly a handy feature to describe and structure long time spans, the Earth's 1999th revolution around the Sun in the anno domini era is really no different than its 2000th revolution.

Only in Gregorian and Julian Calendars

In fact, looking at other calendar systems, it becomes clear how ambiguous year numbers are. For example, year 2001 marked the beginning of the 3rd millennium in the Gregorian calendar only. Other calendars, such as the Jewish calendar, the Islamic calendar, and the Hindu calendar, use completely different year numbers. So, while the Gregorian calendar is the system officially used around the world, this goes to show that our year count is nothing more than a random fabrication, which is ultimately based on the ideas and religious fervor of a 6th-century monk.

What's more, Dionysius Exiguus based the beginning of year BCE 1 in the Julian calendar, the predecessor of today's calendar system, on a religious event—the birth of Jesus—which not only lacks astronomical relevance but is also based on religious lore and, as such, a rough estimation at best.