The Julian calendar was introduced in 46 BC by Julius Caesar and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years. Hence the Julian year is on average 365.25 days long.
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 the Gregorian calendar.
Although the Julian calendar remained in use into the 20th century in some countries and is still used by many national Orthodox churches, it has generally been replaced for civil use by the modern Gregorian calendar. Orthodox Churches no longer using the Julian calendar typically use the Revised Julian calendar rather than the Gregorian calendar. The United Kingdom's driver and vehicle licencing agency (DVLA) still use the Julian calendar when recording periods of vehicle ownershipcitation needed].
Reform was required because too many leap days are added with respect to the astronomical seasons on the Julian scheme. On average, the astronomical solstices and the equinoxes advance by about 11 minutes per year against the Julian year, causing the calendar to gain a day about every 134 years. While Hipparchus and presumably Sosigenes were aware of the discrepancy, although not of its correct value, it was evidently felt to be of little importance. However, it accumulated significantly over time, and eventually led to the reform of 1582, which replaced the Julian calendar with the more accurate Gregorian calendar.