Julian and Gregorian Calendars

Background
The most-used calendar in 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.

The previous calendar in Europe was the Julian Calendar, instituted in 46 BC and named after Julius Caesar. The old Julian Calendar assumed the earth went around the sun in exactly 365.25 days. For this calendar to follow the earth’s movement, this rule was used - every year that was divisible by 4 was made a leap year of 366 days, otherwise it was a standard year of 365 days. In actual 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 calendar was 10 days early.

The Catholic Church was very concerned because the celebration of Easter was figured from the spring equinox. And the spring equinox was now happening 10 days earlier than it should. 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 chopped 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 other countries around the world).

The British Empire changed to the Gregorian Calendar in 1752. Because of these changes, there is some uncertainty for the dates between 1 January and 25 March in the years from 1582 until 1752 in the old British Empire. To avoid any confusion, write the date with both years' numbers. For example - 14 February 1699/1700. At the time it would have been 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.

Julian to Gregorian calendar changes by country or region
The following list attempts to give the year of conversion from the Julian to the Gregorian calendar for many countries and their constituent states, where necessary. Some countries converted early and in their entirety, whereas other countries, such as Switzerland, The Netherlands, and Germany had provinces/cantons that converted at different times. This is because these countries have both Catholic and Protestant areas. Catholics adopted the Gregorian calendar very early, whereas most Protestant areas did not. An attempt has been made to list the conversion years of all constituent states, but this has not been possible in each case, as some states are absent from all the lists of works consulted.

This list has been compiled from various sources (see the list of works consulted at the bottom of the page). The reader will notice that there are sometimes more than one date for conversion for one particular jurisdiction. This is due to several factors. First, sometimes the sources do not agree and most do not give sources. In such cases, it might be impossible to determine the correct date of conversion. Second, a jurisdiction may have made a conversion when the Julian date ended at the end of a year and the Gregorian date took effect in the new year. Some sources list the previous year as the year of conversion, whereas others list the new year as the year of conversion. Third, parts of some jurisdictions may have converted, whereas other parts may have converted later. This is particularly true in Switzerland. Finally, a jurisdiction may have converted to the Gregorian, then back the Julian, then back to the Gregorian (e.g. Groningen). In such cases, the researcher is advised to consult all the sources listed below.

Year of adoption of the Gregorian Calendar. This means that the year given is the beginning year that the country/province/canton began using the Gregorian calendar and when you should start using the Gregorian feast day converter. Before that year, use the Julian converter. For example, Albania used the Julian calendar until 1912, when it switched to the Gregorian. It should be noted that this chart lists only the year, not the date, of conversion. For the exact date (which will be important!), the researcher is again advised to consult the sources listed below.

Ancestor Search has published a helpful chart showing when countries and regions changed from Julian to Gregorian.

Fourmilab.ch has created a converter that converts dates from a variety of calendars, including Julian and Gregorian.

Another tool that can help with Julian and Gregorian dates, especially for Germany, is GenTools6, available as a free download from www.gentools6.de.