  After the study of non-inertial frames in special relativity with emphasis on
the problem of clock synchronization (i.e. of how to define 3-space), an
overview is given of Einstein canonical gravity in the York canonical basis and
of its Hamiltonian Post-Minkowskian (PM) linearization in 3-orthogonal gauges.
It is shown that the York time (the trace of the extrinsic curvature of
3-spaces) is the inertial gauge variable describing the general relativistic
remnant of the clock synchronization gauge freedom. The dark matter implied by
the rotation curves of galaxies can be explained with a choice of the York time
implying a PM extension of the Newtonian celestial frame ICRS.
