Up until 1582, the Julian calendar was used worldwide. Pope Gregory says, I have to solve this problem. Brings in Jesuit priests. In so doing, the Jesuits figure out there are 10 too many days in the calendar. That's how many days had accumulated. Every century is the time you take away a leap day that might have otherwise been there. The last time a century year had a leap day was the year 1600. Just a few years after the Gregorian calendar was put into place. It's one of the most accurate calendars ever devised. And this is one of the many reasons why this calendar is used worldwide.