Calendar introduced under Julius Caesar in 45 BC (actually finalized by Augustus), based on the Egyptian 365-day solar year with a leap day every four years. Early Romans misapplied the rule as 'every three years' because they counted from one; Augustus corrected it and renamed two months after Julius and himself. The Julian calendar drifts relative to the tropical year and was eventually superseded by the Gregorian reform in 1582, though some regions stayed on it into the 20th century. Astronomers still use it for multi-century calculations to avoid the messy adoption history of the Gregorian rules.