Terrestrial Dynamical Time. the independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00sTAI, the value of TDT was exactly 1977 January 1.0003725 d. The unit of TDT is 86 400 SI seconds at mean sea level. For practical purposes TDT= TAI + 32.184 s. (See Barycentric Dynamical Time; dynamical time; International Atomic Time.)