What calendar did Julius Caesar introduce in 45 BC?

Julius Caesar introduced the Julian calendar in 45 BC. Designed with the astronomer Sosigenes of Alexandria, it replaced Rome’s increasingly inaccurate Republican calendar, whose officials had allowed the seasons and civil dates to drift apart.

The Julian system established a 365-day year divided into 12 months, with a leap day added every fourth year. This produced an average year of 365.25 days, a major improvement over the earlier system. To bring the calendar back into alignment with the solar year, Caesar also inserted extra days into 46 BC, making it exceptionally long and sometimes called the “year of confusion.”

The calendar remained the standard in much of Europe for more than 1,600 years. It was later replaced for civil purposes by the Gregorian calendar, introduced by Pope Gregory XIII in 1582 because the Julian leap-year rule was slightly too long. The Julian calendar therefore should not be confused with the older Roman calendar or the later Gregorian reform.