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The Julian Calendar

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A statue of the Roman emperor Julius Caesar Augustus.


The Julian calendar was introduced by Julius Caesar in 45 BCE (Before Common Era) and replaced the Roman calendar.

See the Julian calendar

The Julian calendar has a regular (common) year of 365 days divided into 12 months with a leap day added to the month of February every four years (leap year). This made the Julian year 365.25 days long on average, and needless to say, this extra .25 day caused several issues.

A new solar calendar

The Roman calendar was very complicated. It required a group of people to decide when days should be added or removed in order to keep the calendar in track with the seasons, which are marked by equinoxes and solstices in the calendar.

In order to create a more standardized calendar, Julius Caesar consulted with an Alexandrian astronomer named Sosigenes and created a more regulated civil calendar based entirely on the Earth's revolutions around the sun.

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

Introducing Leap Years

The Roman calendar consisted of 12 months for a total of 355 days. The new Julian months were formed by adding ten days to the pre-Julian Roman year of 355 days, creating a Julian year of 365 days. Two extra days were added to the months Ianuarius (January), Sextilis (August) and December, while one extra day was added to Aprilis (April), Iunius (June), September and November.

The Months of the Year

The Julian calendar introduced the Leap Year every 4 years.

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

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

Too many days

The Julian calendar introduced an error of one day every 128 years, which meant that every 128 years the tropical year shifts one day backwards with respect to the calendar. This made the method for calculating the dates for Easter inaccurate. The solution to this error was to replace the Julian calendar with the Gregorian calendar in 1582 in nearly all countries.

Why are some days missing in the 1752 calendar?

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 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.


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