From Malcolm Longair's Galaxy Formation Second Edition pages 249-250 when discussing the cosmological principle we have the following.

"A key step in the development of these models was introduced by Hermann Weyl in 1923 of what is known as *Weyl's postulate* (Weyl, 1923). To eliminate the arbitrariness in the choice of coordinate frames, Weyl introduced the idea that, in the words of Hermann Bondi (Bondi, 1960):

'The particles of the substratum (representing the nebulae) lie in space-time on a bundle of geodesics diverging from a point in the (finite or infinite) past.'

The most important aspect of this statement is the postulate that the geodesics, which represent the world lines of galaxies, do not intersect, except at a singular point in the finite, or infinite past. Again, it is remarkable that Weyl introduced this postulate *before* Hubble's discovery of the recession of the nebulae. By the term 'substratum' Bondi meant an imaginary medium which can be thought of as a fluid which defines the overall kinematics of the system of galaxies. A consequence of Weyl's postulate is that there is only one geodesic passing through each point in space-time, except at the origin. Once this postulate is adopted, it becomes possible to assign a notional observer to each world line and these are known as *fundamental observers*. Each fundamental observer carries a standard clock and time measured on that clock from the singular point is called *cosmic time*.

One further assumption is needed before we can derive the framework for the standard models. This is the assumption known as the *cosmological principle* and it can be stated:

'We are not located at any special location in the universe.'"

Colin, this is also a situation in which proper against coordinate time can be applied. However the starting point is the big bang and there are still huge gaps in knowledge about this. What does strike me is that the voids between galaxies should have vastly different coordinate time.