Julian–Gregorian Calendar Converter
Convert dates between the Julian (Old Style) and Gregorian (New Style) calendars, with the weekday, Julian Day Number and Old Style double dating.
Two calendars, one reform
The Julian calendar, introduced in 46 BC, has a leap year every four years, making its average year 365.25 days, about 11 minutes longer than the solar year. By the 16th century the error had built up to ten days, and Pope Gregory XIII’s reform of 1582 removed them: in Catholic countries, Thursday 4 October 1582 was followed by Friday 15 October. The Gregorian calendar skips the leap day in century years not divisible by 400, so 1700, 1800 and 1900 were leap years in the Julian calendar but not in the Gregorian, and the gap grew to 11 days from March 1700, 12 from March 1800 and 13 from March 1900. It will become 14 in March 2100. The weekday was never interrupted: the day after Thursday was always Friday.
When each country changed
Spain, Portugal, Italy and Poland changed in 1582, France a few weeks later, and most Catholic German states soon after. Protestant states held out longer: much of Protestant Germany changed in 1700, Britain and its American colonies in 1752, when Wednesday 2 September was followed by Thursday 14 September, Sweden in 1753, Japan adopted the Gregorian calendar in 1873, Russia in 1918 and Greece in 1923. Russia’s October Revolution of 25 October 1917 Julian took place on 7 November Gregorian. Before converting, check which calendar the place of record was using at the time; a date from Boston in 1700 is Julian, while one from Montreal in 1700 is Gregorian.
The start of the year and double dating
England and its colonies also started the legal year on 25 March, Lady Day, until 1752. A baptism on 11 February 1731 in an English register is, by the modern reckoning of years from 1 January, in 1732. George Washington’s birth is the standard example: 11 February 1731 Old Style, written by many contemporaries as 11 February 1731/32, is 22 February 1732 in the Gregorian calendar, a Friday. Genealogists write such dates with both years to avoid ambiguity. The calculator expects years counted from 1 January and shows the double date for any Julian date between 1 January and 24 March up to 1752.
Other famous differences
William Shakespeare died on 23 April 1616 by the Julian calendar used in England, which is 3 May 1616 in the Gregorian calendar; Miguel de Cervantes died in Spain, already Gregorian, on 22 April 1616: a day apart on paper, 11 days apart in fact. A common mistake is to convert every pre-1752 English date by adding 11 days; before March 1700 the difference was 10 days. The Julian Day Number, a continuous day count used by astronomers, gives an unambiguous way to compare dates in any calendar: 1 January 2000 Gregorian is day 2,451,545.
How to use it
- Choose which calendar the date you have is written in.
- Enter the day, month and year, counting the year from 1 January.
- Read the date in the other calendar, the weekday and the Julian Day Number.
- Use the Old Style double date when recording English and colonial dates before 1752.
Questions from the record office
How do I convert a Julian date to Gregorian?
Add the difference between the calendars at that date: 10 days from 15 October 1582 to 28 February 1700, 11 days to 28 February 1800, 12 to 28 February 1900, and 13 from 1 March 1900 to 28 February 2100. The calculator applies the correct difference automatically through the Julian Day Number.
When did Britain and America switch to the Gregorian calendar?
In September 1752. Wednesday 2 September 1752 was followed by Thursday 14 September, removing 11 days. At the same time, the start of the legal year moved from 25 March to 1 January, which had already happened from 1 January 1752.
What does a date like 1731/32 mean?
It is an Old Style double date. Before 1752, England and its colonies began the legal year on 25 March, so dates from 1 January to 24 March belonged to the previous year. 11 February 1731/32 is 1731 by the old year and 1732 by the modern one.
Why was Washington born on 22 February if records say 11 February?
Because he was born on 11 February 1731 Old Style, in the Julian calendar with the year starting on 25 March. Converted to the Gregorian calendar with the year starting in January, that is 22 February 1732, the date celebrated today.
Disclaimer: Genealogy results apply published conventions and inheritance averages to the values you enter. They are not proof of a relationship, parentage or descent, and not legal, medical or genetic advice. Several relationships share each DNA figure, and calendars and spellings varied by place and time: check every conclusion against original records, and consider the privacy of living relatives before sharing it.