Why are there two calendars?
The Julian calendar, introduced by Julius Caesar in 45 BC, adds a leap day every fourth year. That makes its average year about 11 minutes too long, and by the 1500s the calendar had drifted ten days away from the seasons. In 1582 Pope Gregory XIII introduced the Gregorian calendar, which skips three leap days every 400 years: century years are only leap years if they divide by 400.
How far apart are they?
| Period | Gregorian ahead by |
|---|---|
| March 1500 – February 1700 | 10 days |
| March 1700 – February 1800 | 11 days |
| March 1800 – February 1900 | 12 days |
| March 1900 – February 2100 | 13 days |
When countries switched
| Country | Last Julian day | First Gregorian day |
|---|---|---|
| Italy, Spain, Portugal, Poland | 4 October 1582 | 15 October 1582 |
| France | 9 December 1582 | 20 December 1582 |
| Great Britain, Ireland and colonies | 2 September 1752 | 14 September 1752 |
| Sweden | 17 February 1753 | 1 March 1753 |
| Japan | – | 1 January 1873 |
| Russia | 31 January 1918 | 14 February 1918 |
| Greece | 15 February 1923 | 1 March 1923 |
That is why the Russian “October Revolution” of 25 October 1917 is remembered on 7 November. For British dates, which also changed the start of the year, use the Old Style to New Style converter.
Common questions
Is the Julian calendar still used?
Yes. Several Orthodox churches still use it to set religious festivals, which is why Orthodox Christmas falls on 7 January.
What is the Julian Day Number?
A continuous count of days used by astronomers and historians. It makes converting between calendars simple, and the converter shows it for every date.
When will the gap become 14 days?
On 14 March 2100 (Gregorian), because the Julian calendar has a 29 February 2100 and the Gregorian calendar does not.