(ET). It was designed for continuity with ET, and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations). The JPL ephemeris time argument T_eph is within a few milliseconds of TT.

TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich)