> all the Pope Gregory the 13th did was account for leap years by adding in an extra day every few years to keep the year aligned with the astronomical year. T

> all the Pope Gregory the 13th did was account for leap years by adding in an extra day every few years to keep the year aligned with the astronomical year. The Julian calendar has leap days every four years. What Pope Gregory the 13th did was remove it from every 100th year (except those divided by 400, hence why 2000 was a leap year). Otherwise the difference between the Gregorian and Julian calendars would be far more than the 12-13 days they are now.