In astronomy, a Julian year (symbol: a) is a unit to measure time. It has exactly 365 days of 86,400 seconds each. That 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 a Julian year measures duration rather than designates date, the 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 (for which, "see Year").