Julian Calendar Converter
Convert a Gregorian (New Style) date to the Julian (Old Style) calendar, or a Julian date to the Gregorian one. Pick a country and period to see which calendar its documents used on that day, the double year English records carry before 1752, and a table of when each country switched. Runs entirely in your browser — nothing is uploaded.
Pick a date to convert.
When countries switched to the Gregorian calendar
| Place | Last Julian day | First Gregorian day | Days dropped |
|---|---|---|---|
| Spain, Portugal, Papal States and most Italian states | 4 October 1582 | October 15, 1582 | 10 |
| France | 9 December 1582 | December 20, 1582 | 10 |
| Denmark–Norway and Protestant Germany | 18 February 1700 | March 1, 1700 | 11 |
| Great Britain, Ireland and the colonies | 2 September 1752 | September 14, 1752 | 11 |
| Sweden and Finland | 17 February 1753 | March 1, 1753 | 11 |
| Bulgaria | 31 March 1916 | April 14, 1916 | 13 |
| Russia (Soviet decree) | 31 January 1918 | February 14, 1918 | 13 |
| Romania | 31 March 1919 | April 14, 1919 | 13 |
| Greece | 15 February 1923 | March 1, 1923 | 13 |
Many places switched in stages — German, Dutch and Swiss states, for example, changed between 1582 and 1812 — so check the particular town or state in your source.
How to Use the Julian Calendar Converter
- 1. Choose the direction. Gregorian → Julian opens on today's date. For Julian → Gregorian, enter the day, pick the month and type the year exactly as your source gives it.
- 2. Pick a place if you know it. Choosing a country tells you which calendar its people were using on that day, so you can tell whether a date in a letter or register is Old Style or New Style.
- 3. Read the result. You get both dates, the weekday (the same in both calendars), the gap between them in days, the ISO date, and a dual date in the form 2 September 1752 O.S. / 13 September 1752 N.S.
- 4. Check the table. Below the result, the table lists the last Julian and first Gregorian day for nine places, and how many days each one dropped.
Why There Are Two Calendars
The Julian calendar, introduced by Julius Caesar in 45 BCE, adds a leap day every four years, which makes its average year 365.25 days. The true year is about 365.2422 days, so the Julian calendar slipped roughly one day every 128 years against the seasons. By the 16th century the spring equinox had drifted to about 11 March. Pope Gregory XIII's bull Inter gravissimas (24 February 1582) fixed this: it removed ten days and changed the leap-year rule so that century years are leap years only if divisible by 400. That gives 1600 and 2000 a leap day but not 1700, 1800 or 1900.
Because the Gregorian rule skips three leap days every 400 years, the gap between the calendars grows. It was 10 days in 1582, 11 from March 1700, 12 from March 1800 and 13 from March 1900, and it becomes 14 days on 1 March 2100 (Gregorian). The converter works the gap out for every date. Dates before 1582 in the Gregorian column are proleptic: no one wrote them then.
Old Style, New Style and Dual Dating
Historians write O.S. (Old Style) for a Julian date and N.S. (New Style) for a Gregorian one. A dual date gives both, which is common for events between countries that switched at different times. The October Revolution happened on 25 October 1917 O.S. in Russia, which is 7 November 1917 N.S. Isaac Newton was born on 25 December 1642 O.S., which is 4 January 1643 N.S.
England and its colonies added a second wrinkle. Until 1751 the legal year began on 25 March (Lady Day), so 1 January to 24 March belonged to the previous year numerically. Records from those weeks give a double year: George Washington was born on 11 February 1731/32 O.S. — Julian 11 February 1732 by modern year numbering — which is Friday, 22 February 1732 in the Gregorian calendar. The Calendar (New Style) Act 1750 moved the start of the year to 1 January from 1752 and dropped eleven days after 2 September 1752.
| Gregorian dates | Gap (days) |
|---|---|
| 15 October 1582 – 28 February 1700 | 10 |
| 1 March 1700 – 28 February 1800 | 11 |
| 1 March 1800 – 28 February 1900 | 12 |
| 1 March 1900 – 28 February 2100 | 13 |
| 1 March 2100 – 28 February 2200 | 14 |
When Countries Switched
Spain, Portugal, the Papal States and most Italian states changed at once, going from Thursday 4 October to Friday 15 October 1582. France followed in December 1582, Protestant Germany and Denmark–Norway in February 1700, Great Britain and its colonies in September 1752 and Sweden in February 1753. Bulgaria, Russia, Romania and Greece changed between 1916 and 1923, and the Orthodox churches of Russia, Serbia, Jerusalem and others still date their feast days by the Julian calendar, which is why Orthodox Christmas falls on 7 January. The weekday never changed: the day after Thursday 4 October 1582 was Friday, only now called 15 October.
Accuracy and Limits
The conversion uses the fixed-day arithmetic of Reingold and Dershowitz, Calendrical Calculations: The Ultimate Edition (Cambridge University Press, 2018), and the tests check it against an independent Julian Day Number conversion across the whole range, 1 CE to 9999 CE. Years before 1 CE are not supported. The adoption table gives the best-known national switch dates; many regions changed in stages (the Swiss cantons, the Dutch provinces and German states between 1582 and 1812), so confirm the date for a particular town in a local source. Whether a historical writer used Old Style, New Style or both is often unstated; look for O.S., N.S. or a stated year start.
Frequently Asked Questions
- What is the difference between the Julian and Gregorian calendars?
- The Julian calendar has a leap year every four years without exception. The Gregorian calendar skips the leap day in century years not divisible by 400, which keeps it in step with the seasons far longer. Both have the same months and weekday cycle, so the only difference in everyday use is a growing offset of 10 to 14 days since 1582.
- How do I convert an Old Style date to New Style?
- Choose Julian → Gregorian, enter the Old Style day, month and year, and read the Gregorian date. The gap depends on the date, from 10 days in 1582 to 13 days between 1900 and 2100, as the table above shows. The converter applies it exactly, including the Julian leap days (29 February 1700, 1800 and 1900) that the Gregorian calendar lacks.
- Why did Britain lose eleven days in September 1752?
- Britain adopted the Gregorian calendar by the Calendar (New Style) Act 1750, which made Wednesday 2 September 1752 the last Julian day and the next day Thursday 14 September. By 1752 the gap had grown from ten to eleven days, so eleven day names were dropped. Nobody lost eleven days of life; only the labels moved.
- What does a date like 1731/32 mean?
- It is a double year. In England and its colonies before 1752 the year number changed on 25 March, so a date from January to 24 March was written with both years: the old-style year first and the year that would follow by modern reckoning second. A date of 11 February 1731/32 is 11 February 1732 by present-day year numbering.
- Which countries still use the Julian calendar?
- No country uses it for civil life. Some Orthodox churches — Russian, Serbian, Georgian and Jerusalem among them — still use it for Christmas, Easter calculations and saints' days, as do Berber communities for the Amazigh new year. The Revised Julian calendar used by other Orthodox churches matches the Gregorian one until 2800.
- Is the weekday the same in the Julian and Gregorian calendars?
- Yes. The seven-day cycle has run unbroken through the reform: Thursday 4 October 1582 was followed by Friday, which the Gregorian calendar calls 15 October. Only the date labels differ, so a day of the week is a reliable check on a conversion.