I should have read more closely, shouldn't I?
On a cosmological scale, are you saying that a particular rest colour of the CMB represents a moment of simultaneity throughout the universe? - where a "rest colour" is the spectrum of the CMB for an observer who is at rest with the CMB as previously discussed.
Is there anything preventing us from using this as our universal clock (assuming that we could overcome the inaccuracy of measurement)?
The scheme I am thinking of here is this: check that you are inertial (absence of forces), check the CMB in all directions, note the axis of your motion (point of maximum red-shift to point of maximum blue-shift) and then note the colour of the CMB from a direction perpendicular to the axis of motion, take into acount your motion (from the degree of red-/blue-shift) and then you can get the resting CMB colour. Then you know "when" you are in a pretty much absolute sense.
I stress that it is not a question of accuracy here, I am more interested in whether you can talk about a universal "when" even if you can't pin it down to the precise year, or even millenium.