Julian to Gregorian Date Converter for Old Style and New Style Dates

Old American and British records carry dates from two calendars. The Julian calendar, introduced under Julius Caesar in 45 BC, adds a leap day every fourth year without exception. The Gregorian calendar, the one on your wall, skips three of those leap days every four hundred years. So the two drifted apart, and the same day carries two dates: 10 days apart in 1582, 11 in the 1700s, 13 from March 1900 to February 2100. The converter does the arithmetic; the notes below cover what the answer means in a family file.

How to use the converter

Choose the calendar your date is written in, then enter the day, month and year. Enter the year as we count it, starting on 1 January. That matters for dates from 1 January to 24 March before 1752 (see double dates below). The result gives the same day in the other calendar, the gap in days, and, for those early-year dates, the double year a clerk would most likely have written.

How it works

Each date is turned into a Julian Day Number, the running count of days astronomers use, and that number is read back in the other calendar with standard integer formulas. The gap is the difference between the two day counts for the same written date. The rules themselves come from Pope Gregory XIII's reform of 1582 and, for Britain and its colonies, the Calendar (New Style) Act 1750.

Worked example: 11 February 1732

The default is a Julian (Old Style) date of 11 February 1732. The converter returns Gregorian (New Style) 22 February 1732, a gap of 11 days. A Virginia or Massachusetts clerk would have written it as 11 February 1731/32, because the legal year still began on 25 March. Britain and its colonies did not switch until September 1752, when Wednesday 2 September was followed directly by Thursday 14 September.

The date may look familiar: George Washington was born on 11 February 1731/32 by the calendar of his day, and his birthday is kept on 22 February because of exactly this conversion.

Why the calendars differ

A Julian year averages 365.25 days. The solar year, equinox to equinox, is about 11 minutes shorter, and those minutes add up to a full day roughly every 128 years. By the 1500s the spring equinox, which set the date of Easter, had slipped about ten days early. The 1582 reform dropped ten days and changed the leap-year rule: a century year is a leap year only when it divides evenly by 400. So 1600 and 2000 were leap years in both calendars, while 1700, 1800 and 1900 were leap years only in the Julian, and each of those widened the gap by a day.

1582 in Catholic countries, 1752 in Britain

Spain, Portugal, Poland and most of Italy changed in October 1582, when Thursday 4 October was followed by Friday 15 October. France followed that December. Most Protestant German states held out until 1700. Britain and its colonies, including the thirteen that became the United States, waited until 1752. Russia changed only in 1918.

So English colonial records before September 1752 are Old Style, while French and Spanish colonial records from Quebec, Louisiana or Florida were already New Style.

Double dates such as 1731/32

Until 1752 the legal year in England and its colonies began on 25 March, Lady Day, though many people already kept New Year on 1 January. For dates from 1 January to 24 March, careful writers gave both years: 11 February 1731/32, sometimes shortened to 1731/2. The first figure is the legal year, the second the year by January reckoning. The same Act moved New Year's Day to 1 January from 1752, so 1751 was a short year that ran from 25 March to 31 December.

Quaker month numbers

Quakers avoided the pagan names of days and months and numbered them instead: "First Day" for Sunday, "3rd mo." for the third month. Before 1752 the first month was March, so "12th mo. 1731" in a monthly meeting record means February 1731/32, not December. From 1752 the first month is January. Turn the number into a month name first, then convert.

Citing Old Style dates in genealogy

• Copy the date as the source gives it, in day, month, year order with a four-digit year: 11 Feb 1731/32.
• Never shorten a double year to one figure. "1731" alone is ambiguous by a whole year.
• If you convert, keep both and label them: 11 Feb 1731/32 (O.S.), 22 Feb 1732 (N.S.).
• Put two events in the same calendar before you compare them, such as a baptism and a birth, or a will and its probate.
• Work from the original page, not an index. Our guide to reading a 19th-century deed book shows how much an abstract can leave out.

Limits

The converter runs both calendars backward and forward indefinitely, as historians and astronomers do, so it will answer for dates when nobody in a given place used that calendar. It cannot tell you which calendar a particular writer used. That depends on place, date, religion and sometimes stubborn habit. The double year it shows is a guide to usual practice, not a rule; in early 1752 it was habit rather than law. It does not read regnal years ("the twentieth year of the reign of George the Second") or feast days such as Michaelmas, which you need to turn into calendar dates first. Deeds that use them often measure land in rods and chains too; the survey units converter on our workbench handles those.

Frequently asked questions

What is the Julian calendar?

The solar calendar introduced under Julius Caesar in 45 BC, with 365 days and a leap day every fourth year. Britain and its American colonies used it until September 1752.

When did the Gregorian calendar start?

In October 1582, in Catholic Europe, when 4 October was followed by 15 October. Britain and its colonies adopted it in 1752 and Russia in 1918.

Julian calendar vs Gregorian calendar: what is the difference?

The leap-year rule. The Julian adds a leap day every four years; the Gregorian skips it in century years not divisible by 400, which keeps it closer to the solar year.

Why is the gap 10 days in some records and 11 in others?

It grew by a day in 1700, 1800 and 1900, leap years only in the Julian calendar: 10 days from 1582 to February 1700 and 11 days from March 1700 to February 1800.