sentences:  Their names, assigned in 1979, emphasized their dynamical nature or origin, Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT).Both were defined for continuity with ET and were based on what had become the standard SI second, which in turn had been derived from the measured second of ET.During the period 1991–2006, the TDB and TDT time scales were both redefined and replaced, owing to difficulties or inconsistencies in their original definitions.