Exactly 365. 25 days, in which a century (100 years) is exactly 36525 days and in which 1900. 0 corresponds exactly to 1900 January 0. 5 (from the Julian-date system, which is half a day different from civil time or UT). The standard epoch J2000. 0, now used for new star-position catalogues and in solar-system-orbital calculations, means 2000 Jan. 1. 5 Barycentric Dynamical Time (TDB) = Julian Date 2451545. 0 TDB. When this dynamical, artificial "Julian year" is employed, a letter "J" prefixes the year.