Julian and Gregorian Calendars

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The official calendar used in most of the modern world today is the Gregorian Calendar, named after Pope Gregory. It is based on a standard year of 365 days with modifications to keep it consistent with the earth’s movement around the sun. It replaced the old Julian Calendar.

The old Julian Calendar assumed the earth went around the sun in 365.25 days. To keep this calendar consistent with earth’s movement, it followed this rule - Every year that is divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days.

But in actual fact, the earth travels around the sun in 365.2425 days, about 11 minuets shorter than the old Julian Calendar accounted for. This discrepancy accumulated about 3 days short every 400 years. By 1582, the spring equinox was happening 10 days early on 11 March.

The Catholic Church was very concerned because the celebration of Easter was figured from the spring equinox. As a result, on 24 February 1582 Pope Gregory XIII issued a decree (a papal bull) instituting a new calendar.

To returned the spring equinox back to 21 March, the new Gregorian Calendar skipped 10 days from the year. To keep consistent with the earth’s movement around the sun, a new rule was followed - Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years Also, the first day of the year was changed from the 25 March to 1 January.

For example - Benjamin Franklin was born on 6 January 1706. He was 46 years old when the British Empire adopted the Gregorian Calendar in 1752. The date for Benjamin’s birthday did not change just because the calendar changed. He probably had friends with no birthdays that year because they fell during the days that were skipped. Two months after his last birthday, a new year had begun. Then another year began 5 days before this birthday with the new calendar. Two years older from one birthday to the next.

Some countries (usually Catholic) adopted the Gregorian Calendar soon after the Pope issued the decree. Other countries (usually Protestant) ignored the Pope and continued with their own calendars. But gradually the advantages became apparent and most countries one by one have adopted the Gregorian Calendar.

There is some uncertainty with people for the dates between 1 January and 25 March with the years from 1582 till the year the Gregorian Calendar was adopted. To avoid any confusion, write the date with both years numbers. For example - 14 February 1699/1700. At the time it would be considered 1699 according to the Julian Calendar, then in effect. But now it would be considered 1700 according to the Gregorian Calendar. Using the double dating and understand its purpose can be helpful in recording family historical events.

A chart which shows when countries changed from Julian to Gregorian and a converter that translates dates from a variety of calendars are among the tools available to navigate through this maze. Interesting historical reading about how a monarch's reign influenced the English calendar is found in the article Regnal Years in England.

Country Start numbered year
on 1 January
Adoption of
Gregorian Calendar
Denmark Gradual change from
13th to 16th centuries[1]
Venice 1522 1582
Holy Roman Empire 1544 from 1583
Spain 1556 1582
Portugal 1556 1582
Prussia 1559 1700
Sweden 1559 1753
France 1564[2] 1582
Southern Netherlands 1576[3] 1582
Lorraine 1579 1682
Dutch Republic 1583 from 1582
Scotland 1600[4][5] 1752
Russia 1700[6] 1918
Tuscany 1721 1750
Britain and
British Empire
except Scotland
1752[4] 1752


  1. Herluf Nielsen: Kronologi (2nd ed., Dansk Historisk Fællesforening, Copenhagen 1967), pp.48-50.
  2. Le calendrier grégorien en France
  3. Per decree of 16 June 1575. Hermann Grotefend, "Osteranfang" (Easter beginning), Zeitrechnung de Deutschen Mittelalters und der Neuzeit (Chronology of the German Middle Ages and modern times) (1891-1898)
  4. 4.0 4.1 Blackburn & Holford-Strevens (1999), p. 784.
  5. John J. Bond, Handy-book of rules and tables for verifying dates with the Christian era Scottish decree on pp. xvii–xviii.
  6. Roscoe Lamont, The reform of the Julian calendar, Popular Astronomy 28 (1920) 18–32. Decree of Peter the Great is on pp.23–24.