orig. Ugo Buoncompagni
born June 7, 1502, Bologna, Romagna
died April 10, 1585, Rome, Papal States
Pope (1572–85) who promulgated the Gregorian calendar .
After teaching at the University of Bologna, he served as a delegate to the Council of Counter-Reformation , he sought to execute the reform decrees of the council. He compiled the Index librorum prohibitorum and founded several colleges and seminaries, delegating their direction to the Jesuits. Aided by an astronomer and a mathematician, he corrected the errors in the Julian calendar and issued the Gregorian calendar (1582), which was later adopted worldwide.