Per the Julian calendar, each year began on the spring equinox, which usually fell around April 1. Under the Gregorian Calendar, however, the new year started on January 1 .

Because the Julian calendar had 365 and quarter, the calendar was 11 minutes and 14 seconds off every year. More than a millennium later, those superfluous minutes had added up to 10 extra days.

While under the Julian calendar, a day shift accumulated every 129 years, that only happens every 3,333 years under the modern calendar. But that’s a problem for another millennium. About the dates ...