Julian to Gregorian calendar converter: Old Style and New Style dates

Why the two calendars drift apart

The two calendars differ only in their leap year rule. The Julian calendar adds 29 February every fourth year, which makes its average year 365.25 days. The Gregorian calendar skips the leap day in century years not divisible by 400, so 1700, 1800 and 1900 were common years while 1600 and 2000 were leap years. Its average year is 365.2425 days, much closer to the solar year.

Each skipped leap day puts the Julian calendar one more day behind. A Julian date is 13 days earlier than the Gregorian date for the same day today, and the gap will reach 14 days on 1 March 2100 (Julian), because 2100 is a leap year only in the Julian calendar.

The gap grows right after the Julian leap day of a century year. On 29 February 1700 (Julian) it was still 10 days, and on 1 March 1700 it became 11.

The jump of October 1582

The Gregorian calendar was introduced under Pope Gregory XIII in 1582. To bring the date of the spring equinox back to 21 March, ten dates were dropped: Thursday 4 October 1582 in the Julian calendar was followed by Friday 15 October 1582 in the Gregorian calendar. The weekday sequence was not interrupted, so a day keeps its weekday in both calendars.

The converter uses both calendars proleptically, that is, extended backward and forward with their own rules for every date. It does not switch at 1582 or any national date, so you can enter a Julian date from 1066 or a Gregorian date from 2200. For a date from history, first check which calendar the source used. Astronomers usually take Julian dates up to 4 October 1582 and Gregorian dates afterwards, and the Julian Day converter follows that convention.

When countries changed calendars

Each country made the change on its own schedule, so the same event can carry two dates. These switch dates are well documented:

Japan adopted the Gregorian calendar on 1 January 1873, replacing its own lunisolar calendar rather than the Julian one. In Russia, the decree required the Old Style date in brackets until 1 July 1918. The October Revolution of 25 October 1917 (Julian) is on 7 November 1917 in the Gregorian calendar.

Old Style, New Style and double years

Dates in the Julian calendar are marked O.S. (Old Style) and Gregorian dates N.S. (New Style). In England and its colonies the legal year began on 25 March until 1751, so dates from 1 January to 24 March were often written with two years. The Calendar (New Style) Act 1750 made 1 January the start of the year from 1752.

George Washington’s birth shows both effects. The family Bible records 11 February 1731/2: 1731 in the old legal year, 1732 counting from 1 January. In the Gregorian calendar that day is 22 February 1732, the date now celebrated. The converter takes years as starting on 1 January, so enter 1732 for any double-dated year written 1731/32.

Where the Julian calendar is still used

The Russian, Serbian and Georgian Orthodox churches, along with some others, still date their fixed feasts by the Julian calendar. Their Christmas, 25 December (Julian), falls on 7 January in the Gregorian calendar; for Christmas 2026 that is Thursday 7 January 2027. From 2101, after the gap grows to 14 days, it will fall on 8 January.

Most churches of the Greek tradition, as well as the Romanian and Bulgarian churches, adopted a revised Julian calendar in the 20th century and celebrate Christmas on 25 December like Western churches. Orthodox Easter is calculated by separate rules, which this converter does not cover.

Converting by hand

For a date in 1900 to 2100, add 13 days to the Julian date to get the Gregorian date, or subtract 13 to go back. 10 October 2026 (Julian) plus 13 is 23 October 2026 (Gregorian), a Friday in both calendars, Julian Day Number 2461337.

Take care near the end of February in a century year, when one calendar has a 29 February and the other does not, and when the addition crosses a month end. The converter avoids both problems by going through the Julian Day Number, which is the same for the same day in any calendar; how we calculate gives the formulas. For other calendars, such as the Hebrew calendar, the same method applies.

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