The Roman calendar was the calendar used in ancient Rome, which evolved over time. Initially, it consisted of ten months, starting in March and ending in December, totaling 304 days. Later, two additional months, January and February, were added, making it a 12-month calendar. In 45 BCE, Julius Caesar reformed the calendar, creating the Julian calendar. This new system introduced a year of 365 days, with a leap year every four years to account for the extra quarter of a day. The Julian calendar laid the foundation for the modern calendar we use today.