By 46 B.C. the Roman calendar had run 90 days, or nearly three months, behind the seasons. The spring equinox (first day of spring) came in June instead of in March. This situation was corrected by Julius Caesar, who, with the help of the Alexandrian astronomer Sosigenes, reformed the calendar. Caesar added 90 days to 46 B.C., making that year 445 days long. He decreed that the Roman calendar would thereafter have 365 days and that every fourth year would be a leap year with 366 days. The extra day was to be added to February, the shortest month.
The Senate renamed the month called Quintilis (for fifth, from the old calendar beginning with March) July in honor of Julius Caesar. Later, the month called Sextilis (for sixth) was renamed August for Augustus Caesar.
The Julian calendar went into effect in 45 B.C. The Julian year ran only 11 minutes longer than the solar year; after some minor adjustments, the calendar functioned very well. It was introduced gradually throughout western Europe and in time was adopted, at least in modified form, by most of Rome's eastern provinces.