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How Accurate Are Calendars?

The Julian calendar was abolished because it did not reflect the length of a year on Earth accurately. Today's Gregorian calendar does a better job, but is there such a thing as a perfect calendar?

Illustration image

Earth's orbit around the Sun defines year length.

The length of a year on Earth is defined by the time it takes our planet to complete a full orbit around the Sun.


A Year is Not 365 Days Long

The length of a year on Earth is defined by the time it takes our planet to complete a full orbit around the Sun. This is called a tropical year, solar year, astronomical year or equinoctial year, and it is approximately 365.242189 days long. Its length changes slightly over time.

Although a common year has 365 days in today's Gregorian calendar, we regularly add leap days are added regularly to stay in sync with the tropical year. Without leap days, our calendar would be off by one day every 4 years.

The astronomical seasons marked by the equinoxes and solstices would occur at an ever earlier date as time goes by.

Which Years Are Leap Years?

Calendars and Their Accuracy

The table below shows how accurately the different calendar systems reflect the tropical year. There is more information below the table.

CalendarIntroducedAverage Year LengthApproximate Error
Julian calendar45 BCE365.25 days11 min/year (1 day in 128 years)
Gregorian calendar1582 CE365.2425 days27 sec/year (1 day in 3,236 years)
Revised Julian calendar1923 CE365.242222 days2 sec/year (1 day in 31,250 years)
365-day calendar
(no leap years)*
-365 days6 hours (1 day in 4 years)

* The 365-day calendar has never been in use as a civil calendar.

The Julian Calendar

In the Julian calendar, a leap day is added every four years without exception, so an average Julian year is 365.25 days long. The difference between the tropical and the Julian year is about 11 minutes per year, amounting to an error of 1 day every 128 years.

Because of this inaccuracy, the Julian calendar was eventually replaced by the Gregorian calendar.

February 30 was a real date

The Gregorian Calendar

Today's Gregorian calendar uses more elaborate leap year rules, making it far more accurate. However, it is not perfect either. Compared to the tropical year, it is 27 seconds too long, so it is off by 1 day every 3236 years.

So Is There A Perfect Calendar?

The simple answer is no. None of the calendar systems currently in use around the world perfectly reflect the length of a tropical year. However, there are calendar systems that are more accurate than the Gregorian calendar we use today. One of them is the Revised Julian calendar.

Revised Julian Calendar: 10 Times More Accurate

The Revised Julian calendar is the most accurate calendar system ever developed. Here, leap years are observed according to the following rules:

  • Years that are evenly divisible by 4 are leap years.
  • Exception: Years that are evenly divisible by 100 and the remainder is neither 200 or 600 when divided by 900 are not leap years.

This set of rules makes the Revised Julian calendar about 10 times more accurate than our Greogrian calendar, with an error of 1 day in 31,250 years.

Why Don't We Use the Revised Julian Calendar?

First of all, it wouldn't make much of a difference. The offset between the Gregorian and Revised Julian calendars is negligible for many generations to come. The two systems will be in sync until the year 2800, which is a leap year in the Gregorian calendar but not in the Revised Julian calendar. In other words, February 29, 2800 in the Gregorian calendar will be March 1, 2800 in the Revised Julian calendar.

Gregorian calendar for the year 2800

However, the main reason why we do not use the Revised Julian calendar is that it was never meant to be a civil calendar. Rather, it was devised by Serbian scientist Milutin Milanković to reform time reckoning in the Orthodox Church. In 1923, the Greek Patriarch Meletius proposed its adoption. However, only a few national churches followed the directive, so most Orthodox denominations still observe the “original” Julian calendar.

Where Is The Revised Julian Calendar Used?

Today, Milanković's calendar is observed in the following Orthodox churches
Constantinople, Alexandria, Antioch, Romania, Bulgaria, Cyprus, Greece, Albania, Czech Lands and Slovakia, Estonia, Orthodox Churches in America.

Topics: Calendar, Timekeeping, Months, Weekdays

In This Article


Calendar Types

  1. The Gregorian Calendar
  2. The Julian Calendar
  3. The Mayan Calendar
  4. The Chinese Calendar
  5. The Roman Calendar
  6. Switch from Julian to Gregorian
  7. Is There a Perfect Calendar?

Calendars Library

Create Calendar With Holidays

Leap Day Library

  1. Leap Day is February 29
  2. Customs & Traditions
  3. Common Year vs. Leap Year
  4. Famous Birthdays
  5. February 30 Was a Real Date

What is a Leap Year?

Which Months Are The Same


When is the next occurrence of

Which years use the same calendar

Which months are the same

Alternative Leap Years

  1. Bahá'í Leap Year
  2. Chinese Leap Year
  3. Ethiopian leap year
  4. The Hindu leap year
  5. The Iranian leap year
  6. The Islamic leap year
  7. The Jewish Leap Year

Leap Years in Other Calendars

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Equinox and solstice illustration

A Year Is Never 365 Days

The definition of a tropical year, also known as a solar year, astronomical year, or equinoctial year, is the time it takes the Earth to complete a full orbit around the Sun, and it is approximately 365.242189 days long. more

What's a Leap Year?

A leap year has 366 days, as opposed to a common year, which has 365. Nearly every 4 years is a Leap Year, and we add a Leap Day, an extra day on February 29. more