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 indicates a date in the Gregorian calendar.
This notation is used when there might otherwise be confusion
about which date is found in a text. 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 Julia