
 The Time 
  More precisely, it is the time coordinate of a spacetime coordinate system that is centered in space on the barycenter of the solar system, that is nonrotating with respect to distant galaxies, and that tends asymptotically to the proper time of an observer at rest with respect to the coordinate system. 
  Barycentric Dynamical Time, or TDB, is the same as Barycentric Coordinate Time only differing by a constant factor which is chosen so as to make TDB differ from Terrestrial Time by only periodic and Poisson terms. 
  Geocentric Coordinate Time, or TCG, is the same as Barycentric Coordinate Time, except that it uses the Earth's center, rather than the barycenter of the solar system (and an “asymptotic” observer is one free from the gravitational field of the Earth rather than that of all the solar system). 
 www.madore.org /~david/misc/time.html (3957 words) 
