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What is the time argument for using IERS products?
IERS Earth orientation data published in Bulletins A and B are given at 0h UTC (Coordinated Universal Time). The time argument to be used with the IAU2000A precession/nutation model is TT (Terrestrial Time). UTC is defined as TAI (International Atomic Time) + the number of leap seconds since 1972. TT is conventionally realized by TT = TAI + 32.184 s. The error accumulated in TAI since 1977 (i.e. its possible deviation from TT - 32.184 s) can now reach 20 microseconds (which is negligible when computing X and Y, but not the Earth Rotation Angle or UT1).