  In this paper we deduce a quite general formula which allows the relation of
clock rates at two different space time points to be discussed. In the case of
a perturbed Robertson-Walker metric, our analysis leads to an equation for the
comparison of clock rates at different cosmic space time points, which includes
the Hubble redshift, the Doppler effect, the gravitational redshift and the
Rees-Sciama effects. In the case of the solar system, when the 2PN metric is
substituted into the general formula, the comparison of the clock rates on both
the earth and a space station could be made. It might be useful for the
discussion on the precise measurements on future ACES and ASTROD.
