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Here is a cartoon showing the framework of ancient Roman calendar. The ancient Romans took the tropical year to be exactly 365 days long. Their calendar was not very accurate, slipping out of synch with times and seasons by more than 24 days per century. Julius Caesar, out of necessity, reformed the calendar (with the help of Egyptian astronomers) using a tropical year of 365.25 days. This new calendar introduced the ‘leap year,’ and was accurate to about one day in every 128 years. It took much longer for the Julian calendar noticeably to slip, but slip it did, requiring the Gregorian reform late in the 16th century. The tropical year used by Pope Gregory is close to 365.2425 days. The Gregorian calendar is the one in use today. It is accurate to one day in over 3,000 years.
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