The Collegio Romano, now perhaps the most preeminent institution of Jesuit leadership in the world, was initially an informal group of mathematicians focused on Euclidean structures.  Clavius was their articulate spokesman.

To quote Clavius from his Prolegomena, Euclid's Elements was the "sturdiest edifice in the kingdom of knowledge."

His most enduring contribution, however, was the reform of an erroneous calendar.  By the 16th century, it was clear that many religious holidays were no longer accurate.  Clavius formed a group of mathmaticians and astronomers who reconfigured the keeping of time into our current Gregorian calendar.

Scholars argue that in subsequent centuries, the fact the Jesuits and Catholic Church had organized the calendar served the Church well.  During the Reformation, and in centuries of conflict between religions and governments, the Gregorian calendar was never questioned.