The Julian epoch is a way of specifying the date as a year with a decimal part; for example, 1998.5. The unit is the Julian year of 365.25 days and the time scale used is Barycentric Dynamical Time (TDB).
It is often convenient to used the standard epoch, J2000.0, for example, in star catalogs. The prefix "J" is added to distinguish this epoch from the more complicated Besselian epoch (see Besselian year) used before 1984. The standard epoch is defined as J2000.0 = 2000 January 1.5 = 2000 January 1 12h TDB. The date for other epochs can be calculated from this.