The Julian calendar, introduced by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar. It took effect in 45 BC (709 AUC). It was the predominant calendar in most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and superseded by the Gregorian calendar. The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%. The Julian calendar has a regular year of 365 days divided into 12 months, as listed in Table of months. A leap day is added to February every four years. The Jul...

Year zero does not exist in the Anno Domini system usually used to number years in the Gregorian calendar and in its predecessor, the Julian calendar. In this system, the year 1 BC is followed by AD 1. However, there is a year zero in astronomical year numbering (where it coincides with the Julian year 1 BC) and in ISO 8601:2004 (where it coincides with the Gregorian year 1 BC) as well as in all Buddhist and Hindu calendars. Historical, astronomical and ISO year numbering systems Historians The Anno Domini era was introduced in 525 by Dionysius Exiguus (c. 470–c. 544), who used ...