Terrestrial Dynamical Time (TDT) was defined in 1976 by the International Astronomical Union (IAU) to be the counterpart of Barycentric Dynamical Time (TDB) as measured by clocks ticking SI seconds on the surface of the earth. TDT was intended to be a theoretically ideal representation of International Atomic Time (TAI). The distinction between TDT and TAI is that the rules for computing TAI have been changed several times; at one point even the rate of TAI was changed in a discontinuous fashion.
The rates of TDT and TDB are defined such that they deviate only by periodic terms due to the orbital motion of the earth with respect to the solar system barycenter. When the earth is at perihelion in January TDT ticks more slowly than TDB because of the combined effects of special and general relativity. At perihelion the earth moves faster and is also deeper in the sun's gravitational potential well, and both of these effects slow the rate of clocks on the earth. At aphelion in July the opposite holds.
Subsequently the IAU decided that the name of TDT was a misnomer because it did not correspond directly to anything dynamical in the theories of motion for bodies in the solar system. In 1991 the IAU renamed TDT to be simply Terrestrial Time (TT).
