The calendar reform (46 BC)

In 46 BC Caesar reformed the Roman calendar, which the priests had thrown into disorder by inserting months and days at will. He fixed the year at 365 days and added one day every four years. The transitional year had fifteen months.

From this reform comes the Julian calendar, with a 16th-century correction still in use today. He was advised by the astronomer Sosigenes of Alexandria. The year 46 BC lasted 445 days, and the new calendar took effect on January 1, 45 BC.