The Julian Calendar was introduced by Julius Caesar in 45 BC as a reform of the Roman calendar. It replaced the old Roman calendar, which was based on lunar cycles and quickly became inaccurate. The calendar was used in the Western world until it was replaced by the Gregorian calendar in the 16th century. Some synonyms for the Julian Calendar include the Old Style Calendar, the Roman Calendar, the Republican Calendar, and the Julian-style Calendar. While the calendar is no longer in use, it remains a significant part of history and serves as a testament to the efforts of ancient people to measure and order time.