Including subdiurnal and diurnal tides in the calculating the EO terrestrial to celestial transformation may increase the accuracy of that resulting matrix. The Observational EOP data contained in is updated weekly, and in finals2000A.daily, it is updated daily. In each, the observational data for Polar Motion, UT1-UTC, and Celestial Pole Offsets (CPO) are reported at daily intervals at midnight UTC. When one wishes to compute the terrestrial to celestial transformation matrix at times between those midnight intervals, then an interpolation scheme, such as Lagrange interpolation, should be used to estimate the values of Polar Motion and UT1-UTC at those times. In addition, short-period (subdiurnal and diurnal), small-amplitude tidal effects can be included to get an even more accurate estimate of Polar Motion and UT1-UTC at those same times. When the check block labeled "Include Subdiurnal and Diurnal Tides" is checked, these short-period tides are included in the estimates. More details can be found in the reference listed below.
Eanes, R. FORTRAN subroutine ortho_eop.f. Feb 1997
(FORTRAN code can be found at: