Gregorian Vs Julian Calendar. The gregorian calendar adds on the rule that every year. Pope gregory xiii’s reform (see gregorian calendar), proclaimed in 1582, restored the calendar to the seasonal dates of 325 ce, an adjustment of 10 days.

Currently, the julian calendar is 13 days behind the gregorian calendar. It was this calendar which added one extra day in every four years (giving us our ‘leap year’) because it had been calculated that the earth takes.