a calendar introduced by the authority of Julius Caesar in 46 BC, in which the year consisted of 365 days, every fourth year having 366 days. The Julian calendar, proposed by Julius Caesar in 708 Ab urbe condita, was a reform of the Roman calendar. The Julian calendar, proposed by Julius Caesar in 708 Ab urbe condita (AUC) ( 46 BC), was a reform of the Roman calendar. The Julian calendar reformed the ancient Roman calendar and consists of three cycles of 365-day years followed by a 366-day leap year. The Gregorian Calendar, also known as the Western or Christian Calendar, is the most widely used calendar in the world today. Julian calendar, also called Old Style calendar, dating system established by Julius Caesar as a reform of the Roman republican calendar.