Time scale used in orbital computations; TDT is tied to atomic clocks (International Atomic Time, TAI), whereas Universal Time is tied to observations. Prior to 1992, Ephemeris Time (ET) was used in publications of the ICQ/CBAT/MPC; since then, TT has been used. The difference between TDT and UTC in 1994 was 60 seconds (i. E. , UT + 60 seconds = TDT).