- Romulus, one of the founders of Rome, created a calendar that contained 10 months with 304 days, which was considerably shy of the sun's 365-day year. Numa Pompilius, king of Rome, later formulated a religious calendar that added the months of January and February.
- Pompilius' calendar, with the additions of January and February, contained only 355 days. To compensate for the discrepancy between his calendar and the sun's 365-day year, Pompilius added another month called Mercedinus, which was added after February every other year. Roman emperor Julius Caesar later took matters into his own hands and revamped Pompilius' calendar.
- Julius Caesar's calendar contained 12 months, did away with Pompilius' Mercedinus and gave February 29 days plus an extra day every fourth year. Julius Caesar's calendar was slightly modified once more by Caesar Augustus, who took away February's 29th day and added it to August, named in his honor, to match the 31 days contained in July, named in Julius Caesar's honor.