English: TheJulian calendar, a reform of theRoman calendar, was introduced byJulius Caesar in 46 BC, and came into force in 45 BC (709ab urbe condita). It was chosen after consultation with the astronomerSosigenes of Alexandria and was probably designed to approximate thetropical year, known at least sinceHipparchus. It has a regular year of 365 days divided into 12 months, and aleap day is added to February every four years. Hence the Julian year is on average 365.25 days long.

The Julian calendar remained in use into the 20th century in some countries as a nationalcalendar, but it has generally been replaced by the modernGregorian calendar. It is still used by theBerber people ofNorth Africa, onMount Athos and by many nationalOrthodox churches. Orthodox Churches no longer using the Julian calendar typically use theRevised Julian calendar rather than the Gregorian calendar.

The notation"Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which either represents the Julian date with the start of the year as 1 January or a full mapping onto theGregorian calendar.