For years, the Romans had been using a calendar based on lunar cycles. Since each lunar month is 29.5 days, 12 months only adds up to 354 days. But seasons do not follow a lunar cycle, they follow a solar one. A solar year lasts 365 days, 5 hours, 49 minutes. Julius Caesar implemented a solar calendar so that the seasons would occur at the same times every year. Additionally, Caesar made each year 365 days long, with every fourth year being 366 days long (a “leap” year). Unfortunately, each calendar year was still 11 minutes longer than a solar year, a problem that Caesar did not feel was a big concern at the time.