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From the Julian to the Gregorian Calendar

The Gregorian Calendar, also known as the “Western Calendar” or “Christian Calendar”, is the most widely used calendar around the world today.

Bronze statue of Bolognese Pope Gregory XIII in Accursio Palace Bologna

The Gregorian Calendar is named after Pope Gregory XIII.

The Gregorian Calendar is named after Pope Gregory XIII. Bronze statue of Bolognese Pope Gregory XIII in Accursio Palace Bologna.


The Gregorian Calendar's 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 or solar year.

To get the calendar back in tune with astronomical reality, a number of days were dropped in the new calendar, creating irregular months with only 18 days and odd dates like February 30.

When was the Gregorian Calendar introduced?

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.

The Switch around the World

Gregorian Calendar introduction & days dropped
YearCountryDays Removed
1582France (most areas), Italy, Poland, Portugal, Spain, 10 days
1583Austria, Germany (Catholic states), 10 days
1587Hungary, 10 days
1610Germany (Prussia), 10 days
1752Canada (most areas), United Kingdom (and colonies), United States (most areas), 11 days
1872/1873Japan, 12 days
1916Bulgaria, 13 days
1918Estonia, Russia, 13 days
1923Greece, 13 days
1926/1927Turkey, 13 days

* Note:The list shows only a small selection of countries.

Did you know? timeanddate.com's Calendar Generator and Printable PDF Calendars automatically take into account the different dates for the calendar change, meaning that the Julian Calendar is shown by default for years before the switch. It is possible to manually switch to the other calendar system by selecting “Gregorian Calendar” or “Julian Calendar” in the drop-down menu entitled “Country”.

Why did some countries skip more days than others?

The Julian Calendar moves slightly slower than the Gregorian Calendar, introducing an error of 1 day every 128 years. This means that the difference between the two calendar systems increases slowly over time. The papal bull issued by Pope Gregory XIII in 1582 decreed that 10 days be dropped when switching to the Gregorian Calendar. However, many countries chose to introduce the new calendar in later years. The later the switch occurred, the more days had to be omitted. Currently (years 1901 - 2099), the Julian Calendar is 13 days behind the Gregorian Calendar.

The Gregorian Calendar is not perfect either, though somewhat more precise: it is off by only 1 day in 3236 years.

Too many Leap Years

The reason why the Julian Calendar was out of step with the tropical year was the rule it used to define a Leap Year. The Julian Calendar had only one rule to determine whether a year would have 29 days in February instead of 28. If the year could be divided by 4 then it was considered to be a Leap Year. The Gregorian Calendar on the other hand has a more complicated rule for calculating what years will be Leap Years:

  • The year must be evenly divisible by 4;
  • If the year can be evenly divided by 100, it is not a Leap Year, unless;
  • The year is also evenly divisible by 400. Then it is a Leap Year.

These rules result in fewer leap years, minimizing the inaccuracies of the Julian Calendar.

The delay in switching to the Gregorian Calendar 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 considered to be 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 not Leap Years.

(*) Please note: The term “Julian Calendar” is sometimes used to describe a calendar showing day numbers (1 - 365/366). However, this article refers to the Julian Calendar which was in use before the introduction of the Gregorian Calendar.

Topics: Calendar

In This Article


Calendar Types

  1. The Gregorian calendar
  2. The Julian Calendar
  3. The Mayan Calendar - an explanation
  4. The Chinese Calendar
  5. The Roman Calendar
  6. Switch from Julian to Gregorian
  7. Is there a perfect calendar?

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