WordNet (r) 3.0 (2006):

TDT
    n 1: (astronomy) a measure of time defined by Earth's orbital
         motion; terrestrial time is mean solar time corrected for
         the irregularities of the Earth's motions [syn:
         terrestrial time, TT, terrestrial dynamical time,
         TDT, ephemeris time]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

TDT
       Time and Date Table (DVB, UTC)