In astronomy, a Julian year (symbol: a) is a unit of measurement of time defined as exactly 365.25 days of  SI seconds each. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is named. Nevertheless, because an astronomical Julian year measures duration rather than designating a date, this Julian year does not correspond to years in the Julian calendar or any other calendar. Nor does it correspond to the many other ways of defining a year.