The Gregorian reform and Old Style dates

On Christmas Day 1666, Samuel Pepys went to church and dined on roast beef and mince pies. In the reformed calendar most of western Europe already used, that day was January 4, 1667. England still kept the older Julian calendar, ten days behind.

A Julian year of 365¼ days is about eleven minutes longer than the year of the seasons. By the 1500s it had added up to ten days. Easter was reckoned from a spring equinox fixed at March 21, but the real equinox now came ten days earlier. The reform followed a plan by Aloisius Lilius, an Italian physician and astronomer who had died in 1576; a papal commission, with the Jesuit astronomer Christopher Clavius among its members, revised it. Gregory XIII issued it in a bull (papal decree) of February 24, 1582. Ten days were dropped, so Thursday, October 4, 1582, was followed by Friday, October 15. From then on, century years were leap years only if divisible by 400.

Catholic lands took up the new calendar, but Protestant states at first did not. England, which rejected the pope's authority, kept the Julian calendar. In England the year number also changed on March 25, Lady Day, not on January 1, so dates from January 1 to March 24 were often written with two years, as in February 12, 1624/25. Both differences are called "Old Style." George Washington was born on February 11, 1731, Old Style; by the new reckoning, February 22, 1732. Scotland began its year on January 1 from 1600 but kept the Julian days.

Parliament made the change with the Calendar (New Style) Act of 1750. The year 1751 ended on December 31, and 1752 began on January 1. After the Julian leap day of 1700, the gap was eleven days, and these were dropped. September 2, 1752, was followed by September 14. The act kept markets and fairs on the same natural days, so by the new dates they fell eleven days later.

In the library, A diary for every day of the year gives each of Pepys's entries its modern date, with his own Old Style date beside it. Pepys knew the difference; he notes letters from Amsterdam dated in "their style." The headnote to What to do on the farm each month explains that Thomas Tusser's saints' days fell about ten days later in the season than those dates do now.