The Great Calendar Reform: An Historical and Mathematical ExaminationThe calendar introduced by Pope Gregory XIII is now the most widely used civil calendar in the world. The older calendar introduced by Julius Caesar in 45 B. C. underestimated the length of the year by about 11 minutes. As centuries passed, the accumulated error grew. By the late 1500s the Julian calendar was behind by twelve days. Set amid the backdrop of the Reformation and the Renaissance, a time of great schism in the Christian world, the story of