Gregorian Calendar

The slight inaccuracy in the Julian year amounted to a loss of about three-quarters of a day in a century. By 1582 the spring equinox fell on March 11 instead of March 21. Pope Gregory XIII therefore decreed that the day following October 4, 1582, should be the 15th instead of the fifth of the month. To avoid future difficulty, he decreed further that the last year of each century would be a leap year only if that century can be divided by 400. Accordingly, 1600 was a leap year, but 1700, 1800, and 1900 had only 365 days. The year 2000 in the Gregorian calendar was a leap year, with 366 days.

The Gregorian calendar was readily adopted in Roman Catholic countries. Other countries followed, until it became virtually universal as a measure of civil time. By 1752, when England adopted it, the difference between the Julian and Gregorian calendars was 11 days. An act of Parliament therefore provided that the day following September 2, 1752, should be September 14, rather than September 3. At the same time England changed its New Year's Day from March 25 to January 1.

The Gregorian calendar is referred to as New Style (N.S.), and the Julian as Old Style (O.S.). Thus George Washington was born on February 11, 1732 (O.S.). But because he was a young man when the Gregorian calendar was adopted, his birthday is celebrated on February 22 (N.S.).