Today also marks the death of Father Christopher Clavius (1538-1612), the Jesuit mathematician and astronomer who created the Gregorian Calendar, which is still the internationally accepted civil calendar. During his lifetime, he was in close contact with the likes of Galileo and Kepler, who esteemed him highly. Born In Germany, he joined the Jesuits at age seventeen and studied in Portugal and Rome before his ordination. He was then assigned to the Gregorian University in Rome, where he taught mathematics and pursued research. In 1579 he was assigned to compute the basis for a reformed calendar that would stop the slow process whereby Church feast days were drifting relative to the seasons of the year. In a Jesuit curriculum that put a heavy emphasis on philosophy and theology, Clavius pushed for a rigorous mathematics requirement.