Aristarchus of Samos
Some sources state that the Julian calendar was designed by Aristarchus of Samos, although it is not clear where this conclusion originates. Ptolemy III Euergetes, Aristarchus' contemporary, did indeed decree a reform of the Egyptian calendar in 238 BC, but it was never implemented. The reform, however, would have added an extra day (leap day) to the 365-day Egyptian calendar every four years, a feature shared by the Julian calendar.