The Julian and Gregorian calendars have the same months and the same leap day in February. The only difference is the leap year rule. The Julian calendar adds a leap day every four years without exception. The Gregorian calendar skips it in century years not divisible by 400, such as 1900 and 2100. Because of those skipped days, Julian dates now run 13 days behind Gregorian ones: October 27, 2026 in the Gregorian calendar is October 14, 2026 in the Julian calendar.
The two calendars side by side
| Julian calendar | Gregorian calendar | |
|---|---|---|
| Introduced | 45 BC, by Julius Caesar | 1582, by Pope Gregory XIII |
| Leap year rule | Every year divisible by 4 | Divisible by 4, except centuries not divisible by 400 |
| Leap years per 400 years | 100 | 97 |
| Average year | 365.25 days | 365.2425 days |
| Drift against the seasons | 1 day in about 128 years | 1 day in about 3,200 years |
| Used today for | Some Orthodox church calendars | Civil dates almost everywhere |
Why the switch happened
The Julian year is about 11 minutes longer than the tropical year. By the 1500s the accumulated error had moved the spring equinox about ten days earlier, to around March 11. Since the date of Easter is calculated from the equinox, the Church wanted it back near March 21.
The reform itself is described in the Gregorian calendar article.
Gregory XIII's bull Inter gravissimas, issued in February 1582, did two things. It deleted ten days, so that Thursday, October 4, 1582 was followed by Friday, October 15, 1582. The weekdays kept running without a break. And it introduced the century rule so the drift would not return.
Who switched when
Catholic countries followed quickly. Protestant and Orthodox countries took much longer, which is why historical dates can be confusing.
| Country | Last Julian date | Next day (Gregorian) |
|---|---|---|
| Italy, Spain, Portugal, Poland | October 4, 1582 | October 15, 1582 |
| Great Britain and its colonies | September 2, 1752 | September 14, 1752 |
| Russia | January 31, 1918 | February 14, 1918 |
| Greece | February 15, 1923 | March 1, 1923 |
Britain had to drop 11 days rather than 10 because 1700 had been a leap year in the Julian calendar but not in the Gregorian one. The same change moved the British start of the year from March 25 to January 1. That is why George Washington's birth date can be written two ways: February 11, 1731 under the old style, and February 22, 1732 under the new.
Russia's late switch explains why the October Revolution of 1917 is remembered in November. It began on October 25 in the Julian calendar Russia still used, which was November 7 in the Gregorian.
How big the gap is
The difference grows by one day every century year that is a Julian leap year but not a Gregorian one:
| Period | Julian date is behind by |
|---|---|
| 1582 to 1700 | 10 days |
| 1700 to 1800 | 11 days |
| 1800 to 1900 | 12 days |
| 1900 to 2100 | 13 days |
| From March 2100 | 14 days |
The gap changes at the end of February in those century years. That's why Orthodox churches that keep the Julian calendar celebrate Christmas on January 7 in the Gregorian calendar today, and will celebrate it on January 8 from 2101.
What "Julian date" means
The phrase has two unrelated meanings, and neither one is a date in the Julian calendar.
1. The astronomical Julian Day. Astronomers count days continuously from noon Universal Time on January 1, 4713 BC (in the proleptic Julian calendar), so any two moments can be compared by simple subtraction. The count was based on the Julian Period proposed by Joseph Scaliger in 1583. A Julian Day starts at noon, so January 1, 2000 at noon is Julian Day 2,451,545.0 and October 27, 2026 at noon is 2,461,341.0. Astronomers and spacecraft operators also use the Modified Julian Date, which subtracts 2,400,000.5 so that days start at midnight.
2. The ordinal date used in business. In manufacturing, food packaging, the military and many mainframe systems, a "Julian date" is the year followed by the day of the year. October 27, 2026 is day 300 of 2026, so it is written 26300 in the five-digit form or 2026300 in the seven-digit form. February 29, 2028 is 28060.
To read an ordinal date, you need the day number of the first of each month:
| Month starts | Day number (common year) | Day number (leap year) |
|---|---|---|
| January 1 | 1 | 1 |
| February 1 | 32 | 32 |
| March 1 | 60 | 61 |
| April 1 | 91 | 92 |
| May 1 | 121 | 122 |
| June 1 | 152 | 153 |
| July 1 | 182 | 183 |
| August 1 | 213 | 214 |
| September 1 | 244 | 245 |
| October 1 | 274 | 275 |
| November 1 | 305 | 306 |
| December 1 | 335 | 336 |
Add the day of the month minus one. For example, 26300 is in 2026, a common year; October 1 is day 274, so day 300 is October 27.
Related reading
The leap year rule at the heart of all this is explained in leap year rules, and the Roman origins of the month lengths in why February has 28 days. For today's dates, the 2026 calendar and 2027 calendar show the Gregorian year with ISO week numbers.
Frequently asked questions
Only the leap year rule. The Julian calendar has a leap year every four years. The Gregorian calendar skips century years not divisible by 400, which keeps it closer to the solar year.
13 days, from March 1900 until March 2100, when the gap grows to 14 days.
It depends which meaning you need. As an ordinal date, a Julian date is the year plus the day of the year, so October 27, 2026 is 26300. As an astronomical Julian Day, noon on October 27, 2026 is 2,461,341.