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. 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 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 back 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 years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years.

The Pope could not mandate these changes, only make the proposal. 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 adopted the Gregorian Calendar (first European contries and later countries around the world).

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 American colonies were part of the British Empire at the time.) He probably had several 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 just days before this birthday because of the new calendar. Two years older from one birthday to the next.

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 understand its purpose can be helpful in recording historical events.

Here are two links which show when various political entities switched from the Julian to the Gregorian calendar: first chart and secondHere is a converter that converts 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.