The Gregorian calendar
Oct 4 → Oct 15, 1582
The Gregorian calendar began in 1582: Thursday, October 4 (Julian) was followed by Friday, October 15. It keeps 97 leap days in every 400 years and is the civil calendar almost everywhere today.
Today (Gregorian): November 3, 2026Julian calendar: October 21, 2026Difference: 13 days until 2100
The missing days
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 15 | 16 | |
| 17 | 18 | 19 | 20 | 21 | 22 | 23 |
| 24 | 25 | 26 | 27 | 28 | 29 | 30 |
| 31 |
| Sun | Mon | Tue | Wed | Thu | Fri | Sat |
|---|---|---|---|---|---|---|
| 1 | 2 | 14 | 15 | 16 | ||
| 17 | 18 | 19 | 20 | 21 | 22 | 23 |
| 24 | 25 | 26 | 27 | 28 | 29 | 30 |
The underlined day is the first Gregorian date. The weekdays continue without a gap: in 1582, Thursday the 4th is followed by Friday the 15th, and in 1752, Wednesday the 2nd by Thursday the 14th.
Julian and Gregorian date converter
Type in either box; the other follows. Dates are year-month-day. Both calendars are counted through the Julian Day Number, the day count astronomers use, with the algorithms published by the US Naval Observatory.
Why the calendar was reformed
The Julian calendar, introduced by Julius Caesar in 45 BC, has a simple rule: three years of 365 days, then one of 366. Its average year of 365.25 days is about 0.0078 days, some 11 minutes, longer than the tropical year, the cycle of the seasons, which is about 365.2422 days. That adds up to a little over three quarters of a day per century. According to the US Naval Observatory, between Julius Caesar’s time and the 16th century the start of spring shifted from March 23 to March 11.
That mattered most to the Church. The rules for the date of Easter treated March 21 as the date of the spring equinox, and the real equinox kept moving away from it. The reform issued under Pope Gregory XIII did two things. It removed ten dates once, so that the equinox returned to around March 21. It also changed the leap year rule for the future: century years are leap years only when divisible by 400. That leaves 97 leap days per 400 years instead of 100, an average year of 365.2425 days, within half a minute of the tropical year.
The months, their lengths and the leap day on February 29 stayed as they were, and so did the weekday cycle. No days were lost, only date numbers: Thursday, October 4 was followed by Friday, October 15, and the calendar dates in between never happened in the countries that switched that night.
When each country switched
The reform was a papal decree, so the first to adopt it were Catholic countries. Protestant and Orthodox countries took up to three and a half centuries longer, and each switch had to skip more days, because the Julian calendar kept gaining a day in 1700, 1800 and 1900. The table gives the first Gregorian date where the US Social Security Administration records one. The 1582 and 1752 dates come from the US Naval Observatory and from the British Act.
| Year | Date | Where |
|---|---|---|
| 1582 | Thursday, October 4 (Julian) → Friday, October 15 | The first Catholic countries, at the papal reform (dates per USNO) |
| 1752 | Wednesday, September 2 (Julian) → Thursday, September 14 | Great Britain and its colonies, by the Calendar (New Style) Act 1750; the legal year also began on January 1 from 1752 |
| 1873 | January 1 | Japan (solar calendar with Gregorian months) |
| 1912 | November 28 | Albania |
| 1916 | April 14 | Bulgaria |
| 1918 | February 14 | Estonia, Latvia and Russia (in Russia, remote registrars kept the old calendar for up to 10 years) |
| 1918 | December 1 | Former Austro-Hungarian areas of Yugoslavia (Bosnia, Croatia, Dalmatia, Herzegovina, Slavonia, Slovenia, Vojvodina) |
| 1919 | January 15 | Macedonia, Montenegro and Serbia |
| 1919 | April 15 | Romania (the Romanian Church followed on October 1, 1924) |
| 1923 | March 1 | Greece, for civil records (Julian February 15 was followed by Gregorian March 1) |
| 1926 | January 1 | Turkey |
Britain needed 11 days in 1752, not 10, because 1700 had been a leap year under the Julian calendar but not under the Gregorian. The same Act also moved the start of the legal year. Until then, the year in England had begun on March 25, so a date such as February 1750 in an old English record is February 1751 in today’s numbering (1751 itself was cut short and ended on December 31). Historians mark such dates “Old Style” and “New Style”. The terms can mean the calendar change, the New Year change, or both, so read them in context.
How far apart the two calendars are
The gap between the two calendars grows by one day at every century year that the Julian calendar counts as a leap year and the Gregorian does not. In 1600, 2000 and 2400 both have a leap year, so nothing changes. Each new difference begins on the Gregorian date that matches Julian March 1 of that century year:
| Gregorian dates | Julian calendar is behind by | Starts at Julian |
|---|---|---|
| October 15, 1582 to March 11, 1700 | 10 days | October 5, 1582 |
| March 12, 1700 to March 12, 1800 | 11 days | March 1, 1700 |
| March 13, 1800 to March 13, 1900 | 12 days | March 1, 1800 |
| March 14, 1900 to March 14, 2100 | 13 days | March 1, 1900 |
| March 15, 2100 to March 15, 2200 | 14 days | March 1, 2100 |
| March 16, 2200 to March 16, 2300 | 15 days | March 1, 2200 |
| March 17, 2300 to … | 16 days | March 1, 2300 |
So for every date from March 1900 to February 2100, the Julian date is the Gregorian date minus 13 days. That is why churches that keep the Julian calendar for feast days celebrate Christmas, December 25 in their calendar, on January 7 of the civil calendar. From March 2100 that Christmas moves to January 8. At the edges of each span, a Julian leap day has no Gregorian twin. Julian February 29, 1900, which has no Gregorian counterpart with the same name, was Gregorian March 13, 1900. Julian February 29, 2100 will be Gregorian March 14, 2100.
Using the Gregorian calendar today
The Gregorian calendar is the world’s civil calendar, and ISO 8601 dates such as 2026-11-02 are written in it. For dates before 1582, most software extends the Gregorian rules backwards, the proleptic Gregorian calendar, while historians normally quote such dates in the Julian calendar that was in use at the time. A date from before 1752 in an English source, or before 1918 in a Russian one, is usually Old Style. Convert it before you compare it with a modern date or work out its weekday. The converter above does this, and leap years explains the century rule in detail.
Many countries use another calendar alongside the Gregorian. Examples are the Islamic (Hijri) calendar, the Hebrew calendar, Iran’s Solar Hijri calendar and the Japanese era years. Each of these follows its own rules, and dates in them convert through the same day count. Today’s Islamic date shows one of them, and the Julian date explains the astronomers’ day count that shares the Julian name.