Gregorian calendar

The calendar now referred to by the majority of countries, introduced in 1582 by Pope Gregory XIII in his reform of the Julian calendar. With its 365.25 days, the Julian year was 11 minutes and 14 seconds longer than the interval between two consecutive spring equinoxes. Because of the difference accumulated over the centuries, by 1582 it was 10 days out.

To solve this problem, Gregory XIII took ten days from the calendar: Thursday October 4th 1582 was followed by Friday October 15th. As previously, every fourth year becomes a leap year by adding a 29th day to February.

In order to erase three days every 400 years, a century year is no longer a leap year except when divisible by 400. Hence 1600 and 2000 were both leap years whereas 1700, 1800 and 1900 were ordinary years, as will be 2100 and 2400.

The Gregorian calendar is therefore just three days in advance every 10,000 years.

Years the reform was adopted

• 1582: Italy, Spain, Portugal, France, Catholic Netherlands.
• 1584: Austria, Germany, Catholic Switzerland.
• 1586: Poland.
• 1587: Hungary.
• 1610: Prussia.
• 1648: Alsace (Strasbourg in 1682 following its annexation by France).
• 1700: Germany, Switzerland, Protestant Netherlands, Denmark, Norway.
• 1752: England and its American colonies, Sweden.
• 1753: Protestant Switzerland (the last canton was Grisons in 1811).
• 1873: Japan.
• 1912: China.
• 1916: Bulgaria.
• 1918: Russia
• 1919: Rumania, Yugoslavia.
• 1923: Greece.
• 1926: Turkey.

NB: The Orthodox churches continue to refer to the Julian calendar, either:

• totally: Russia, Mount Athos, Serbia, or
• partially: Greece, Rumania