Gregorian Calendar History: From Caesar to the 1752 Switch

The calendar on an American wall today is the Gregorian calendar, a corrected version of the Julian calendar that Julius Caesar introduced. Pope Gregory XIII ordered the correction in 1582, and Catholic countries went straight from Thursday, October 4, 1582 to Friday, October 15. Britain and its American colonies waited 170 years. For them, Wednesday, September 2, 1752 was followed by Thursday, September 14.

That gap matters to anyone reading colonial records, family Bibles or early American letters. A date written in British territory before September 1752 usually follows the Julian calendar, and it may also use a year that began on March 25 rather than January 1. Together, those two differences explain why George Washington’s birthday is February 22, 1732, even though the day was recorded as February 11, 1731.

Before Gregory: the Roman and Julian calendars

The calendar of the Roman Republic had an ordinary year of 355 days. Officials kept it near the seasons by inserting an extra month when they judged it necessary, and by the 40s BC the months had drifted far from their proper seasons.

Julius Caesar replaced that system with a solar calendar designed with help from the astronomer Sosigenes of Alexandria. To pull the months back into place, the year now numbered 46 BC was stretched to 445 days. The new calendar took effect on January 1, 45 BC. It had 365 days in ordinary years and a leap day every fourth year, for an average of 365.25 days.

The start was rocky. Roman priests at first inserted the leap day every third year instead of every fourth, and Augustus later suspended leap days for several years to absorb the surplus. Historians still disagree about when the four-year pattern ran correctly again; AD 4 and AD 8 are both proposed.

Why the Julian calendar drifted

The Julian average is slightly too long. The seasonal, or tropical, year is about 365.2422 days, so a 365.25-day calendar runs about 0.0078 of a day ahead each year. That is roughly 11 minutes a year, or one full day every 128 years.

The error showed up at the spring equinox. Church tradition placed the equinox on March 21 at the time of the Council of Nicaea in AD 325, and the date of Easter was calculated from that fixed day. Over the 1,257 years between 325 and 1582, an error of 0.0078 day a year adds up to about 9.8 days. Gregory’s bull put the drift at “about ten days,” which meant the real equinox was arriving around March 11 while church tables still assumed March 21.

Britain’s Parliament described the same problem when it finally acted. The preamble to its calendar law says the equinox, which fell on or about March 21 at the Council of Nicaea, now arrived on the 9th or 10th of March.

Pope Gregory XIII’s 1582 reform

The core proposal came from Aloysius Lilius, and in 1579 Gregory assigned the Jesuit mathematician Christopher Clavius to the work of recalibrating the calendar. The result was the papal bull Inter gravissimas, dated February 24, 1582. (The bull’s own dateline reads 1581, because of the way the year was counted at the time.)

The bull made two changes:

1. A one-time jump. Ten dates were removed from October 1582, so the day after October 4 was October 15. The weekday cycle was not interrupted: Thursday was followed by Friday.
2. A new leap-year rule. A century year would be a leap year only if divisible by 400. The year 1600 kept its February 29; 1700, 1800 and 1900 lost theirs; 2000 kept it again.

The new rule drops three leap days every 400 years, leaving 97 leap years per cycle and an average year of 365.2425 days. That is still about 26 seconds longer than the seasonal year, so the error takes more than 3,000 years to reach a full day. For the arithmetic behind the rule, see how leap years work. The bull also reset the church’s lunar tables, which control how Easter’s date is calculated.

Italy, Spain, Portugal and Poland changed on the bull’s schedule in October 1582. France followed that December, when December 9 was followed by December 20.

Why Britain waited until 1752

Protestant and Orthodox countries were slow to adopt a reform issued by the pope. Denmark, Norway and the Protestant German states switched in 1700. Britain kept the Julian calendar, and the gap kept growing: 1700 was a leap year under Julian rules but not under Gregorian rules, so by 1752 the two calendars were eleven days apart instead of ten.

Parliament finally passed the Calendar (New Style) Act 1750, often called the Calendar Act of 1751 in American sources. The law gave practical reasons as well as astronomical ones. The old system caused mistakes in dates and complicated dealings with merchants and correspondents abroad, where the new style was already in use.

The act did three things:

• It made January 1 the first day of the legal year, starting with 1752. Until then, the legal year in England and the colonies began on March 25, Lady Day.
• It ordered that the day after September 2, 1752 be counted as September 14, “omitting for that time only the eleven intermediate nominal days.”
• It adopted the Gregorian century rule, naming 2000 as the first century year that would remain a leap year.

What 1752 looked like in the American colonies

The law applied across Britain’s colonies, including the thirteen that later formed the United States. Because the start of the year moved too, the colonies lived through two short years. The year 1751 began on March 25 and ended on December 31, which made it 282 days long. The year 1752 began on January 1, kept February 29 (it was a leap year under the old rules), and lost eleven September dates, for a total of 355 days.

Colonial newspapers prepared their readers. According to the Charleston County Public Library, the South Carolina Gazette printed the full text of the act on January 1, 1752, and on September 1 reminded readers that “Thursday next will be the 14th of September.” Habits lagged behind the law. The library’s history notes that some South Carolinians kept celebrating “Old Christmas” on January 5, which is where December 25 on the Julian calendar landed after the change.

Not every part of today’s United States shared that timeline. Spain and France had used the Gregorian calendar since 1582. Alaska, then a Russian territory, changed calendars only in October 1867, when it became part of the United States.

Old Style and New Style: George Washington’s birthday

Historians label dates as Old Style (O.S.) or New Style (N.S.). In British and colonial records, Old Style usually means two things: the Julian calendar and a year that began on March 25. Dates between January 1 and March 24 were often written with both years, such as “1731/2,” to show both counts.

Washington’s birth shows both adjustments. The National Park Service notes that if you had asked Washington for his birthday, he would have answered February 11, 1731. Converting it takes two steps:

1. Fix the year. That February fell near the end of the year that began on March 25, 1731. Counted from January 1, it belongs to 1732.
2. Add the calendar difference. In 1732 the Julian calendar ran 11 days behind the Gregorian calendar, so February 11 becomes February 22.

The result is Friday, February 22, 1732. The weekday is the same in both calendars because the change renamed dates without skipping days of the week. The first public celebration of his birthday took place at Valley Forge in 1778, and the day was first observed as a federal holiday in 1880. Since 1971 the holiday has fallen on the third Monday in February, and its official federal name is still Washington’s Birthday, as our Presidents’ Day guide explains.

How to convert an Old Style date, step by step

Use this method for a date from a British or colonial source written before September 14, 1752.

1. Confirm the place. A 1720 letter from Boston uses the Julian calendar; a 1720 letter from Paris uses the Gregorian one. The calendar depends on where the writer lived, not on the date alone.
2. Check the year. If the date falls between January 1 and March 24 and shows a single year, it may be an Old Style year. Add one to get the modern year when the context confirms it.
3. Add the right number of days for the Julian date:

Julian date	Days to add
October 5, 1582 to February 28, 1700	10
February 29, 1700 to February 28, 1800	11
February 29, 1800 to February 28, 1900	12
February 29, 1900 to February 28, 2100	13

4. Keep the weekday. A Thursday in the old calendar is still a Thursday after conversion.

A worked example: a colonial deed dated January 10, 1744/5 means January 10, 1745 by modern year numbering, still on the Julian calendar. Adding 11 days gives Thursday, January 21, 1745 in the Gregorian calendar.

Old Calendars’s day of the week calculator applies Gregorian rules to every date, so convert an Old Style date before checking its weekday. The same caution applies when you research what happened on a past date in colonial newspapers, or count the days between two dates on either side of 1752.

Later adopters and the calendar today

Sweden switched in 1753, after an earlier attempt that had gone wrong. Japan adopted the Gregorian calendar on January 1, 1873, and Turkey on January 1, 1927. In Russia, January 31, 1918 was followed by February 14, a jump of 13 days, and Greece changed in 1923. China adopted the calendar in 1912, although it was not used across the whole country until 1949.

Many Orthodox churches still follow the Julian calendar for religious feasts. For Gregorian years 1901 through 2100, December 25 on the Julian calendar falls on January 7, which is why some churches celebrate Christmas in January while most American churches mark Christmas on December 25.

The Gregorian calendar also underpins the international date standard ISO 8601 and the ISO week numbers used in business planning. Every calendar on this site, from colonial-era conversions to the current year, rests on the leap-year rule that Gregory’s astronomers wrote in 1582 and Parliament copied into law in 1751.