Julian to Gregorian Date Converter
Convert a date between the Julian calendar (Old Style) and the Gregorian calendar (New Style) in either direction, with the day of the week, the 10- to 13-day gap for the century, and notes on when each country changed calendars and on years that began on 25 March.
Last updated: October 2026
Converted Date
13 September 1752 (Gregorian), a Wednesday
Formula below · 3 sources (aa.usno.navy.mil, legislation.gov.uk, fourmilab.ch) · Updated Oct 2026
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About this calculator
In Britain and its American colonies, Wednesday 2 September 1752 was followed by Thursday 14 September 1752: the Calendar (New Style) Act 1750 dropped eleven days to bring the country from the Julian calendar onto the Gregorian one that Pope Gregory XIII had introduced in 1582, when Catholic states went from 4 October straight to 15 October. This converter turns a date in one calendar into the same day in the other, in either direction, and names the day of the week.
Why the calendars differ
The Julian calendar, introduced by Julius Caesar in 45 BC, makes every fourth year a leap year, giving an average year of 365.25 days, about 11 minutes longer than the tropical year. By the 16th century the error had reached ten days, and the spring equinox fell on 11 March instead of 21 March. The Gregorian reform kept the leap-year rule but dropped it for century years not divisible by 400: 1700, 1800 and 1900 are not leap years, 2000 was. Because the Julian calendar keeps adding a 29 February in those years, the gap between the calendars grows by one day in each such century: 10 days from 1582 to 28 February 1700 (Julian), 11 days to 1800, 12 days to 1900, 13 days from 1 March 1900 (Julian) to 2100, and 14 days after that.
How the conversion works
Each date is turned into its Julian Day Number, the count of days since 1 January 4713 BC in the Julian calendar, with the standard astronomical algorithms, and that number is turned back into a date in the other calendar. The weekday follows from the day number (0 = Monday in that count). Dates before 1582 are converted to the proleptic Gregorian calendar, the Gregorian rules extended backwards, which is what historians and astronomers use for them.
Old Style and New Style years
Before 1752 the English legal year began on 25 March, not 1 January, so a document dated 11 February 1731 (Old Style) was written in what we call February 1732; historians write it 11 February 1731/2. The converter assumes the year you enter already begins on 1 January, so for an English date between 1 January and 24 March before 1752, add one to the year first. George Washington was born on 11 February 1731 Old Style, which is 22 February 1732 New Style.
When each country changed
Italy, Spain, Portugal and Poland changed in October 1582, France in December 1582, the Catholic German states and the Netherlands in 1582–1583, Protestant Germany, Denmark and the Dutch Protestant provinces in 1700, Britain and its colonies in 1752, Sweden in 1753, Japan in 1873, Russia in 1918 (so the "October Revolution" of 25 October 1917 Old Style took place on 7 November New Style), Greece in 1923 and Turkey in 1926; the Orthodox churches still set Easter and many feasts by the Julian calendar. The table below lists the dates.
When countries moved from the Julian to the Gregorian calendar
Scroll sideways to see all columns →
| Country or region | Last Julian day | First Gregorian day | Days dropped |
|---|---|---|---|
| Italy, Spain, Portugal, Poland | 4 October 1582 | 15 October 1582 | 10 |
| France | 9 December 1582 | 20 December 1582 | 10 |
| Catholic German states, Austria, Catholic Netherlands | 1583–1584 (various) | 10 | |
| Protestant German states, Denmark, Norway, Dutch Protestant provinces | 18 February 1700 | 1 March 1700 | 11 |
| Great Britain, Ireland and the colonies (incl. the future United States) | 2 September 1752 | 14 September 1752 | 11 |
| Sweden | 17 February 1753 | 1 March 1753 | 11 |
| Japan | 31 December 1872 | 1 January 1873 | 12 |
| Russia (Soviet decree) | 31 January 1918 | 14 February 1918 | 13 |
| Greece | 15 February 1923 | 1 March 1923 | 13 |
| Turkey | 18 December 1926 | 1 January 1927 | 13 |
How to use
Choose the direction, then enter the year, month and day as they appear in the source. The default converts 2 September 1752 (Julian) and returns 13 September 1752 (Gregorian), a Wednesday: the 11-day gap of the 18th century. Entering 25 October 1917 Julian gives 7 November 1917 Gregorian, a Wednesday (13-day gap). To go the other way, choose Gregorian to Julian: 15 October 1582 Gregorian returns 5 October 1582 Julian: the reform replaced the day after 4 October with 15 October. The tiles show the gap between the calendars at that date and the Julian Day Number.
Frequently asked questions
How many days are between the Julian and Gregorian calendars?
It depends on the century: 10 days from 1582 until the end of February 1700, 11 days from March 1700, 12 from March 1800, 13 from March 1900, and 14 from March 2100. The gap grows by a day in every century year that the Julian calendar treats as a leap year but the Gregorian does not (1700, 1800, 1900, 2100), and stays the same across years divisible by 400 (1600, 2000).
When did Britain and America switch to the Gregorian calendar?
In September 1752, under the Calendar (New Style) Act 1750: 2 September was followed by 14 September, and the legal year had already been moved to begin on 1 January from 1752 instead of 25 March. The change applied to Great Britain, Ireland and the colonies, including the future United States.
What do Old Style (O.S.) and New Style (N.S.) mean?
Old Style is a date in the Julian calendar, New Style in the Gregorian. In England the terms also cover the start of the year: Old Style years began on 25 March, so a date between 1 January and 24 March carries a year number one lower than in New Style reckoning. Historians write such dates with both years, for example 11 February 1731/2.
Was 1900 a leap year?
Not in the Gregorian calendar: century years are leap years only when divisible by 400, so 1700, 1800 and 1900 were ordinary years while 1600 and 2000 were leap years. In the Julian calendar every fourth year is a leap year, 1900 included, which is why the gap between the calendars grew from 12 to 13 days in March 1900.
Why did Russia's October Revolution happen in November?
Russia still used the Julian calendar in 1917. The Bolsheviks seized power on 25 October 1917 Old Style, which was 7 November 1917 in the Gregorian calendar used by the rest of Europe; Soviet Russia adopted the Gregorian calendar in February 1918, dropping 13 days.
Can I convert dates before 1582?
Yes. For dates before the reform the converter gives the proleptic Gregorian equivalent, the Gregorian rules extended backwards, which is the convention used by astronomers and in many databases. Contemporary documents from before 1582 are always Julian, so convert only when you need to align a date with a modern or astronomical calendar.
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Sources & references
- U.S. Naval Observatory — Introduction to Calendars (Julian and Gregorian leap-year rules, the 1582 reform)
- legislation.gov.uk — Calendar (New Style) Act 1750 (the British change of September 1752 and the 1 January year start)
- Fourmilab — Calendar Converter (Julian and Gregorian calendar rules, 4 to 15 October 1582, British adoption in 1752)
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