The Difference Between the Julian and Gregorian Calendar: Old and New Style

01One leap rule creates the widening gap

Both calendars normally use twelve familiar months and add a leap day after February. Their rules diverge at century years:

The Julian average is 365.25 days. The Gregorian average is 365.2425. A difference of 0.0075 day per year looks tiny, but every Gregorian century exception increases the date offset by one full day.

The calendars agreed more closely in earlier centuries. When the Gregorian reform first took effect, it omitted ten date labels. Gregorian 1700, 1800 and 1900 then skipped leap days that Julian reckoning retained. The gap became eleven, twelve and thirteen days.

In 2100 the Gregorian calendar will skip another Julian leap day. Julian dates from March 1 onward will then require a fourteen-day addition.

02Julian-to-Gregorian offset table

Use this table for dates after the listed Gregorian boundary. To express a Julian date as the Gregorian date naming the same physical day, add the offset. To move Gregorian to Julian, subtract it.

The boundary detail prevents a common error. Gregorian 1700 had no February 29, but Julian reckoning did. A single “add eleven to every Julian date in 1700” rule therefore mishandles January and February; the new offset applies to Julian source dates beginning March 1.

This arithmetic is not the same as legal adoption. Britain used Julian dates until 1752 even though the mathematical Gregorian equivalent existed. Historians often preserve the date a contemporary source actually used, label its style, and supply a converted date only when helpful. Converting every pre-1582 event into a proleptic Gregorian date without saying so can manufacture anniversaries no witness recognized.

03O.S. and N.S. can carry two corrections

O.S. means Old Style; N.S. means New Style. In many European contexts, the labels distinguish Julian and Gregorian dates. Russian documents around the 1917 revolutions provide famous examples: an event named for “October” Old Style occurred in November New Style.

British and colonial records add another layer. Before 1752, England's legal year began on March 25, Lady Day, even though January 1 was widely treated as a seasonal New Year. Under that legal convention, the day after December 31, 1750 was January 1, 1750; the numbered year did not become 1751 until March 25.

A writer using New Style might mean Gregorian day numbering, a January 1 year start, or both. Context controls. Editors should state their convention instead of assuming the initials always perform one operation.

Scotland had already changed its numbered year to begin January 1 in 1600 while retaining the Julian calendar for day dates. That example proves the two reforms are logically separable: a community can change the year label without yet changing the month and day.

04Why a record says 1750/51

Double dating disambiguates the weeks between January 1 and March 24. A British record dated February 10, 1750 under the legal March-year belongs to 1751 under January-year numbering. An editor may write February 10, 1750/51: the first year records the contemporary Old Style legal label, and the second tells a modern reader which January-based year contains it.

Do not add the day offset merely because a slash appears. The slash solves the year-start problem. Converting the calendar date is a separate step. In Britain's eighteenth century, that second step adds eleven days.

Genealogists should preserve the original form in a transcription, record the calendar and locality, then place any normalized date in a clearly labeled field. Otherwise a careful 1750/51 can be flattened into a confident but unexplained 1751, making it impossible for the next researcher to retrace the decision.

Britain's 1752 transition combined both changes: January 1 became the legal year opening, and September 2 was followed by September 14.

05Washington's birthday performs both steps

Washington's family Bible records his birth on February 11, 1731/2. Virginia was a British colony, so the Julian calendar and March 25 legal year applied. Modernizing it requires two independent moves.

First, change the year style. February falls before March 25, so Old Style 1731 becomes January-year 1732. The date is now February 11, 1732 with the calendar still Julian.

Second, convert the day style. The eighteenth-century offset was eleven days, so Julian February 11 becomes Gregorian February 22. The result is February 22, 1732.

The National Archives describes the combined shift as 376 labels—365 from the year designation plus eleven from the calendar date—but Washington lived one continuous day. He did not acquire a second birth.

This worked example is useful because it prevents a wrong shortcut: changing 1731 to 1732 without moving the day, or adding eleven days while leaving the Old Style year untouched.

06Day 4 is not an Old Style converter

Day 4 does not display Julian dates, O.S./N.S. notation or historical locality rules. Use a scholarly converter and inspect the source's dating convention for archival work. The app handles a different pairing: today's civil Gregorian date beside today's Day 4 sacred year-day.

That limitation is worth stating early. A calendar app cannot infer whether “February 1750” came from England, Scotland, Russia or an editor who already normalized it.

07Common questions