The Julian Calendar, proposed (allegedly) and instituted by Julius Caesar in 46 B.C. was a reform of the old Roman Calendar based on a 10 month (March-December) calendar year started at the time of the Roman Empire founder, King Romulus (whenever THAT was). The mean calendar year equals 365 days, 5 hours, 49 minutes, 12 seconds; abbreviated 365.2423 days long. The reform added January and February, making a 12 month year of roughly 30 days each and shortened the average year by 0.0075 days...

The Calendar Conundrum