Complex sentence: The Gregorian calendar was a reform of the Julian calendar instituted in 1582 by Pope Gregory XIII, after whom the calendar was named, by papal bull "Inter gravissimas" dated 24 February 1582.
Split into simpler sentences: The Gregorian calendar was a reform of the Julian calendar. It was instituted in 1582 by Pope Gregory XIII, after whom the calendar was named, by papal bull "Inter gravissimas" dated 24 February 1582.