The Roman calendar's long dysfunction — Julius Caesar found in 46 BC that the official calendar was three months out of alignment with the solar year — reflects the intersection of religious conservatism, political manipulation, and genuine astronomical ignorance that characterized Roman temporal management.

The pre-Julian Roman calendar was a lunar calendar with a 10-month year (hence September, October, November, December meaning seventh through tenth month), later modified to 12 months but still requiring periodic insertion of an intercalary month (Mercedonius) to stay roughly aligned with the solar year. The pontiffs who managed intercalation used it for political purposes — extending years when their allies were in office, shortening them when their enemies held power — until the calendar became essentially meaningless.

Julius Caesar's reform, implemented with the help of the Egyptian astronomer Sosigenes, adopted the Egyptian solar calendar of 365 days with a leap year every four years. The Julian calendar was close enough to the solar year (it overestimates by 11 minutes per year) to function well for centuries. By the 16th century, the cumulative error had grown to 10 days, necessitating the Gregorian reform of 1582.

The Roman naming of months — January (Janus), March (Mars), May (Maia), June (Juno) — and the renaming of Quintilis and Sextilis after Julius Caesar and Augustus give the modern calendar a direct Roman inheritance. The seven-day week (named for the sun, moon, and five visible planets) was adopted by Rome from Eastern traditions during the early empire and transmitted to the medieval West through Christianity.