After various positions as judge, vice-chancellor, priest, and legate to the Spanish King, he ascended to the papal position. Once there, he set aside his worldly concerns and sought to reform the Catholic religion. One of his best known acts was the reformation of the calendar, producing the Gregorian calendar.
After United Earth Starfleet Captain Erika Hernandez and other Columbia officers stuck on the Caeliar city-ship of Axion were thrown back in time, Hernandez thought of various Earth people and events as they returned to their original position in the timeline. Despite their exile, Erika still rendered time using the Gregorian calendar. (ST - Destiny novel: Mere Mortals)