  In special and general relativity the synchronization convention of distant
clocks may be simulated with a mathematical definition of global non-inertial
frames (the only ones existing in general relativity due to the equivalence
principle) with well-defined instantaneous 3-spaces. For asymptotically
Minkowskian Einstein space-times this procedure can be used at the Hamiltonian
level in the York canonical basis, where it is possible for the first time to
disentangle tidal gravitational degrees of freedom from gauge inertial ones.
The most important inertial effect connected with clock synchronization is the
York time {}^3K(\tau, \sigma^r), not existing in Newton gravity. This fact
opens the possibility to describe some aspects of {\it darkness} as a
relativistic inertial effect in Einstein gravity by means of a Post-Minkowskian
reformulation of the Celestial Reference System ICRS.
