Calendar Adoption

Calendar Changes when adopting the Gregorian Calendar

While Spain, Portugal, the Polish Lithuanian Commonwealth and most of Italy adopted the "Gregorian" Calendar in October 1582, not all countries and locations adopted the refined calendar at the same time. For example, the American Colonies did not adopt the "Gregorian" Calendar until September 1752 (170 years later). Therefore, October 1582 would appear as it had before, but September 1752 drops out 11 days (1 more than in 1582 due to accumulating more error). Turkey did not adopt the Gregorian calendar until 1926.

In Great Britain and the British Colonies, the New Year (Civil Year) started on March 25th before 1752. Scotland switched to January 1st in 1600, but continued to use the Julian calendar. They adopted the Gregorian calendar at the same time as the rest of Great Britain (September 1752). Most other countries started using January 1 as the start of a New Year as far back as 1522. Great Britain and the colonies were the last to switch.

Dates using the Julian calendar are called Old Style dates (abbreviated OS), and Gregorian dates as New Style date (abbreviated NS). It is very common to convert Old Style dates to new Style dates, especially where the date is celebrated, such as George Washington's birthday. When a date is converted to the Gregorian format, the OS and NS designation should be used to alleviate any ambiguity.

If using the dates for a calculation of age, such as the number of days lived, is based on a birth date before 1752 but a death date after 1752, keep in mind that without knowing what country the date is based on, it is not guaranteed to produce accurate results. Even for U.S. dates, not all calculating programs will subtract the 11 days in September 1752.

The start of year switch is important in age calculations when using Old Style dates because dates for January, February, and the first 24 days in March would use the previous year. In addition if the birth date is in January, February, or the first 24 days of March, using the recorded year (which will be the previous year), will cause the result to be a full year off. In this case, the birth date should use the Old Style month and day, but use the next year as the birth year (the same year as April through December). Examples of this dilemma can be found by calculating the number of days that U.S. Presidents lived. The first four presidents were born before 1752, and George Washington and James Madison were born before March 25, thus for their Old Style birth year, use the next year instead.

Some examples of when countries adopted the Gregorian Calendar:

Months with unusual number of days (in increasing years)

• 21: October 1582 had 21 days (Catholic countries, including Spanish controlled territories in what is now the U.S. states of Arizona, California, Florida, Nevada, New Mexico, and Texas)
• 21: December 1582 had 21 days (France, Loraine, some Dutch provinces, Southern Netherlands, U.S.A. Mississippi Valey)
• 25: December 1582 had 25 days (Denmark, Flanders)
• 27: January 1583 had 27 days (Denmark, Flanders)
• 21: January 1583 had 21 days (Holland)
• 18: February 1700 had 18 days (Denmark that then included Norway and parts of Germany)
• 20: July 1700 had 20 days (Gelderland)
• 20: December 1700 had 20 days (Utrecht and Overijssel)
• 20: January 1701 had 20 days (Friesland and Groningen)
• 30: February 1712 had 30 days (Sweden)
• 19: September 1752 had 19 (Great Briton, American Colonies, Eastern Seaboard, Oregon, Washington)
• 17: February 1753 had 17 days (Sweden and Finland)
• 20: October 1867 had 20 days (Alaska)
• 15: February 1918 had 15 days (Russia)
• 15: February 1923 had 15 days (Greece)
• 18: December 1926 had 18 days (Turkey)

Months with unusual number of days (in increasing days)

• 15: February 1918 had 15 days (Russia)
• 15: February 1923 had 15 days (Greece)
• 17: February 1753 had 17 days (Sweden and Finland)
• 18: February 1700 had 18 days (Denmark that then included Norway and parts of Germany)
• 18: December 1926 had 18 days (Turkey)
• 19: September 1752 had 19 (Great Briton, American Colonies, Eastern Seaboard, Oregon, Washington)
• 20: July 1700 had 20 days (Gelderland)
• 20: December 1700 had 20 days (Utrecht and Overijssel)
• 20: January 1701 had 20 days (Friesland and Groningen)
• 20: October 1867 had 20 days (Alaska)
• 21: October 1582 had 21 days (Catholic countries, including Spanish controlled territories in what is now the U.S. states of Arizona, California, Florida, Nevada, New Mexico, and Texas)
• 21: December 1582 had 21 days (France, Loraine, some Dutch provinces, Southern Netherlands, U.S.A. Mississippi Valey)
• 21: January 1583 had 21 days (Holland)
• 25: December 1582 had 25 days (Denmark, Flanders)
• 27: January 1583 had 27 days (Denmark, Flanders)
• 30: February 1712 had 30 days (Sweden)

Months with unusual number of days by year

Months with unusual number of days by Month

Prior to 45 BC, March, June, August, November also had different number of days than what they have today. Therefore, March and May are the only months that have never had their number of days changed. Note that August has had 31 days since it was named August (changed from Sextils to August by Augustus Caesar, who also added the 31st day).