Patriarch of the Gregorian calendar. Pope Gregory XIII, born Ugo Boncompagni, was head of the Catholic Church and ruler of the Papal States from May 13th, 1572 to his death in 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally accepted civil calendar to this day.

Once in the chair of Saint Peter, Gregory XIII dedicated himself to reform of the Catholic Church. He implemented the recommendations of the Council of Trent. He mandated that cardinals reside in their sees without exception, and designated a committee to update the Index of Forbidden Books. Gregory XIII was also the patron of a new and greatly improved edition of the Corpus juris canonici. He was renowned for fierce independence; some confidants noted that he neither welcomed interventions nor sought advice. The power of the papacy increased under him, whereas the influence and power of the cardinals substantially decreased.

Gregory XIII was a generous patron of the recently formed Society of Jesus throughout Europe, for which he founded many new colleges. The Roman College of the Jesuits grew substantially under his patronage and became the most important center of learning in Europe for a time.

Pope Gregory XIII is best known for commissioning the Gregorian calendar, initially authored by the doctor/astronomer Aloysius Lilius and aided by Jesuit priest/astronomer Christopher Clavius, who made the final modifications. This calendar is more accurate than the Julian calendar, which treats each year as 365 days and 6 hours in length.