Suppose we have a helium atom.  The nucleus is composed of four baryons, two dressed up as neutrons and two dressed up as protons.  The whole assembly is held together by the strong nuclear force, one of the four fundamental forces of nature, the others being gravity, electrostatic and weak nuclear.  The baryons have existed since the Big Bang but their assembly into a helium nucleus has been a relatively recent event.  Prior to coming together in the nucleus the baryons had different relativistic histories so it would be very unlikely that they would all be the same age. 

Since no particle can participate in an interaction at any time other than its now, the ongoing participation of all four baryons in the same interaction holding the nucleus together means that now for all of them must have the same time coordinate, regardless of their different ages.   The same analysis applies to the electrons interacting with the nucleus.  Also the analysis applies to atoms interacting in an object so the Earth and everything on it is constrained to have every particle’s now time coordinate be identical.

Of course we have demonstrated that the now of a satellite in orbit and of the Earth share the same time coordinate by the fact that the GPS does indeed work.  We can confirm that this extends to the Moon by noticing that the Moon’s orbit about Earth is an ellipse.  If now on the Moon was at a different time than now on Earth, one of the bodies would be in the other’s future and therefore unavailable for the gravitational interaction, as well as unable to scatter light to its partner.

It is beginning to seem that all the particles in every object that we can see or otherwise sense must share the same now time coordinate.  Perhaps we have come the long way around to Newton’s third law: “For every action there is an equal and opposite reaction”.  If objects are out of phase in time, the action and reaction cannot be always equal and opposite.

How about galaxies far, far away beyond our observable universe? Since every observers observable universe must overlap with that of her distant friend, there is no place for a transition to take place.  Every particle in the universe experiences now at the same time coordinate and I am betting it is the age of the universe. 

Even though all their nows are in phase there is not a single now covering all the particles in the universe.  Each particle’s now is a point on its own world line.  That may seem like an unnecessary complication but the line of simultaneity paradox dictates that each particle have its own now.