Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. Beginning with the ephemeris for 1984, TDB has been the independent variable of time used for calculating the motions of bodies in the solar system. TDB is a timescale that increments uniformly in a reference frame co-moving with the barycenter of the solar system. On the average the rate of TDB matches the rate of atomic clocks ticking SI seconds on the surface of the earth.
TDB is the direct successor of Ephemeris Time in that the values of physical constants, notably the Gaussian gravitational constant, match the traditional values from pre-relativistic days. Terrestrial Dynamical Time (TDT) was defined at the same time as TDB.
It was soon realized that TDB was not well defined because it was not accompanied by a general relativistic metric and because the exact relationship between TDB and TDT had not been specified. Furthermore, because the length of the TDB second is determined by clocks on earth (as opposed to the barycentric reference frame itself) it disagrees with the SI second that would be determined by a clock at rest in the frame. As a result, in 1991 the IAU refined the notions of timescales by creating Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG) as replacements for TDB.
Despite IAU recommendations that TCB be used for all further calculations of solar system ephemerides, as of 2002 TDB continues to be used. For practical purposes the only difference between TDB and TCB is that TCB ticks faster, but this rate difference means that physical constants have different values in TCB than they do in TDB. Changing software from the traditional TDB values to the recommended TCB values will require considerable effort.
