By 1582, the discrepancies in the Julian calendar were not interfering with timekeeping, but they were beginning to infringe on dates of the church’s ecclesiastical holidays. The powerful Catholic Church was not amused: Pope Gregory XIII, on the advice of several of his astronomers, decided to reposition days, striking out the excess ten days that had accumulated on the then-present-day calendar. Thus, October 4, 1582 was followed by October 15, 1582.

To fix the extra-days problem, the pope made sure that the last year of each century would be a leap year, but only when it is exactly divisible by 400. That means that three leap-years are suppressed every four centuries; for example, 1900 was not a leap year, but 2000 was a leap year. (Today, the Gregorian calendar “rules” state that every year divisible by four is a leap year, except for years that are both divisible by 100 and not divisible by 400.)

Some countries eliminated the ten extra days, starting “fresh” with the Gregorian calendar. But not everyone agreed with the new calendar, especially those who distrusted and disliked the Catholic Church. Eventually, by 1700, those who had not changed their calendars had collected too many extra days. In 1752, the English Parliament decreed that eleven days would be omitted from the month of September. England and its American colonies began to follow the Gregorian calendar, with most other countries following close behind. It is now the standard calendar used around the world.