How to use it
- Choose Julian date to Gregorian or Gregorian date to Julian in Convert.
- Type the date in Date, in year-month-day form, in the calendar you chose.
- Read Same day in the other calendar and Difference in days.
- Use Difference by period to see how the gap grows, and When countries adopted the Gregorian calendar for the years of change.
Why the two calendars differ
The Julian calendar adds a leap day every fourth year. It assumes a year of 365.25 days, while the solar year is shorter by 0.0075 days (about 11 minutes), so the calendar drifts against the seasons.
The Gregorian calendar, set out in the papal bull Inter gravissimas of 24 February 1582, keeps the four-year rule but omits the leap day in three century years out of four: a year divisible by 100 is a leap year only if it is also divisible by 400.
How the difference in days is worked out
The difference is the number of days between the two labels of one day, Gregorian minus Julian. It is 10 from 15 October 1582 to 28 February 1700, rises by one day in each century year that the Gregorian calendar skips, and is 13 from 1 March 1900 to 28 February 2100.
For dates from 1 March of a year Y, the difference is floor(Y / 100) − floor(Y / 400) − 2, with Y in astronomical years, so 1 BC is year 0. This is the secular difference given in the Gregorian calendar article.
difference = floor(Y / 100) − floor(Y / 400) − 2Countries that changed later
Great Britain and its colonies changed in 1752: Wednesday 2 September was followed by Thursday 14 September. Russia changed by decree of 24 January 1918, which made 14 February follow 31 January. The Ottoman fiscal calendar was realigned on 1 March 1917, when 16 February Julian became 1 March Gregorian. Turkey adopted the Gregorian calendar for general use on 1 January 1926.
The table lists the first Gregorian day in each country. Many countries changed the start of their year as well, so a year number can change at a different date from the calendar.
What this converter does not cover
- Old-style and new-style double dates, such as 10/21 February 1750/51, which some records use to show the year change as well as the calendar change.
- Dates before 4713 BC, which lie before JDN 0; the page does not convert them.
- Calendars other than the Julian and the Gregorian, such as the Hijri or the Hebrew calendar. Other tools on this site convert those.
Worked example: 4 October 1582 in the Julian calendar
Choose Julian date to Gregorian and type 1582-10-04. In 1582 the Julian calendar was 10 days behind, so the same day is 14 October 1582 in the Gregorian calendar, and the difference is 10.
The day after 4 October (Julian) is 15 October (Gregorian), the first day of the Gregorian calendar in the countries that changed at once. Converting 15 October 1582 (Gregorian) gives 5 October (Julian).
| Julian date | Same day, Gregorian | Difference (days) |
|---|---|---|
| 4 October 1582 | 14 October 1582 | 10 |
| 5 October 1582 | 15 October 1582 | 10 |
Frequently asked questions
Why are there two calendars?
The Julian calendar drifted against the seasons by about one day in 128 years, so the Gregorian calendar corrected its leap years in 1582. Countries changed at different times, and some kept the Julian calendar into the 20th century.
How do I know which calendar a historical date uses?
Check the country and the year of the record. Dates before the change in that country are usually Julian, and the adoption table gives the first Gregorian day for the countries it lists.
Why does the difference grow over time?
Each century year that is not divisible by 400, such as 1700, 1800 and 1900, is a leap year in the Julian calendar but not in the Gregorian one. Each of those adds one day to the difference.
Why were ten days dropped in October 1582?
The Julian calendar had drifted by about ten days from the astronomical year by then, so the Gregorian reform removed the dates from 5 to 14 October 1582 in the countries that adopted it at once.
Is the date I type sent to a server?
No. The conversion runs in your browser. Nothing you type is stored or sent to us, and you can use the page without an account.
Sources
- Wikipedia: Gregorian calendar (the reform, the difference table and the secular difference)
- Wikipedia: Adoption of the Gregorian calendar (dates by country)
- Wikipedia: Rumi calendar (the 1917 change of the Ottoman fiscal year)
- US Naval Observatory: Julian Date Converter (the 1582 change)
Figures and text last checked on Tuesday, November 10, 2026.