Terrestrial Dynamical Time (TDT) is dynamical time for geocentric phenomena which replaced Ephemeris Time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984. TDT is independent of the variable rotation of the Earth, and the lengths of the tropical year and synodic month are generally defined in days of 86,400 seconds of international atomic time. TDT is used as the time scale of ephemerides for observations from Earth's surface, and differs from international atomic time (TAI) by an offset

which varies slowly with time.