Caesar's calendar year of confusion lasted 445 days
Julius Caesar fixed Rome's drifting seasons with a 365-day solar year and a leap day every four years. The scheme ruled the West for sixteen centuries before its small yearly error snowballed into a ten-day Easter problem.
The Julian calendar assigns 365 days to ordinary years and 366 to leap years on a strict four-year cycle, averaging 365.25 days. Proposed in 46 BC and effective from 1 January 45 BC, it replaced a chaotic Roman lunisolar system where pontiffs could lengthen years for political allies. Between 1901 and 2099, Gregorian dates run thirteen days ahead of Julian ones.
Caesar's reform needed a dramatic reset. Year 46 BC—the "last year of confusion"—was stretched to 445 days by inserting two extra months between November and December atop an existing intercalary February, realigning 1 March with 1 January. Ten days were distributed across Julian months: two each to January, Sextilis, and December; one each to April, June, September, and November. February kept twenty-eight days except in leap years.
The leap day was the bissextile—"twice sixth"—created by doubling 24 February rather than adding 29 February. Astronomer Sosigenes of Alexandria, likely the principal designer, combined Roman months, Egypt's fixed 365-day year, and the Greek 365¼-day estimate. Varro used the stable calendar in 37 BC to date the four seasons—impossible eight years earlier.
The Julian year exceeds the tropical year of about 365.2422 days by roughly eleven minutes annually, gaining about three days every four centuries—or one day every 128 years. Gregory's 1582 reform trimmed centurial leap days to fix Easter drift. Orthodox churches still use Julian dates for major feasts; many adopted Gregorian civil calendars in the early twentieth century.
Source: Julian calendar
