In ancient times the Roman Calendar had fallen into disarray and had become almost unusable for the purposes of the Roman Empire. In 46 BC, Julius Caesar gave Sosigenes of Alexandria the task of designing an easy-to-use and exact calendar. Sosigenes assumed that the year had a length of 365¼ days and worked out the leap year rule, by which three common years should be followed by one leap year, the former having 365 days each, while the latter should have 366 days. The months were no longer determined by the Moon's phases and were given lengths of 30 or 31 days, excepting February, which was given 28 days in common years and 29 in a leap year.