Dynamical time for barycentric phenomena (TDB) which replaced ephemeris time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. The difference between terrestrial dynamical time and TDB is due to variations in the gravitational potential around the Earth's orbit combined with velocity terms, and is always less than 2 milliseconds. TDB is used as a time scale of ephemerides referred to the barycenter of the solar system.