Julian and Gregorian Calendars

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. Though not perfect, the Gregorian Calendar will take 3300 years before being one day off. It replaced the old Julian Calendar instituted in 46 BC and named after Julius Caesar.

The old Julian Calendar assumed the earth went around the sun in 365.25 days. For this calendar to follow the earth’s movement, this rule was used - every year that is divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. In fact, the earth travels around the sun in 365.2422 days, about 11 minutes shorter than the old Julian Calendar. This discrepancy accumulated about 3 days short every 4 centuries. 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 return the spring equinox to 21 March, the new Gregorian Calendar skipped 10 days from the year. Also, a change already in progress was validated - the first day of the year was changed from the 25 March to 1 January. But most importantly, 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 centurial years that are not divisible by 400. This means that the year 2000, being exactly divisible by 400, was a leap year, but 2100, 2200, and 2300, although falling in the 4-year cycle, are not leap years because they are not evenly divisible by 400.

The Pope could not mandate these changes, only make the proposal. Some countries (mostly Catholic) adopted the Gregorian Calendar soon after the Pope issued the decree. Other countries (mostly Protestant) ignored the Pope and continued with their own calendars. But gradually the advantages became apparent and most countries adopted the Gregorian Calendar (first European countries and later countries around the world).

There is some uncertainty for the dates between 1 January and 25 March in 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-year dating and understanding its purpose can be helpful in recording 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.