Terrestrial Time

time The current clock, and calendar, of astronomy, using the second of an atomic clock, with no adjustment for the vagaries of the natural day, etc. Until 1991 called Terrestrial Dynamical Time (TDT), it succeeded the similar Ephemeris Time (ET), based on a fixed-size second of ¹/₈₆₄₀₀ an idealized day, in 1984. It is synchronized with TAI, offset permanently by the 32.184 s that ET was in advance of TAI at the moment of succession. See time, also Universal Time, which, unlike TAI and TT (the preferred name now being the shorter one), allows for adjustment to maintain harmony with the observed day.