Gregorian Calendar Invented. When the gregorian calendar firmly established january 1 as the beginning of its year, it was widely referred to as the new style calendar, with the julian the old style calendar. The gregorian calendar was instituted by pope gregory xiii in 1582 and quickly adopted by much of catholic, but not protestant, europe.
Pope gregory xiii invented the gregorian calendar in 1582; Lilius’s correction, introduced during the papacy of gregory xiii, meant at last that the year was.