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Calendars and Dates

In 46 B.C., Julius Caesar decreed a calendar with 365 days divided into 12 months to be used in Rome. The calendar introduced a leap day every four years to account for those extra quarter days it takes to circle the sun. And it would have been a good calendar if a year real were 365.25 days long, but a year is 365.24 days long. So the Julian calendar added too many leap years. By the 14th century, the spring equinox had slipped 10 days to March 11 instead of March 21. That threw off the date of Easter, which falls on the Sunday after the first full moon after the spring equinox.

In 1582, Pope Gregory XIII convened a panel to fix the calendar. Because the Julian calendar was so far ahead  of the season, 10 days were chopped out of October, so October 4, 1582, was followed by October 15, 1582. To prevent the calendar from getting ahead of itself again, the scientists proposed to hobble some of the leaps. Now leap years occur in years divisible by 4, except for years ending in 00. If a century is divisible by 400, it is a leap year. So 2000 was a leap year, but 2100 will not be a leap year. The Gregorian calendar also established January 1 as the start of the year. The change was quickly implemented by Catholic countries, but rejected in Protestant regions. It was gradually accepted by most of Europe between 1699 and 1701.

England, Ireland and the American colonies didn't start using the Gregorian calendar until 1752. By that time, the Julian calendar had slipped another day, so 11 days had to be cut out of September 1752 to compensate. Confusion is sometimes encountered when attempting to calculate a pre-1752 birth date based on a post-1752 age at death.