You'd need to state a reference. For example, you can say that the light that reaches us from M31, or vice versa, is 2.3 million years older than the light from a local source (because we are 2.3 million light years from M31).
But you could never "synchronize" an event between two locations that are so far apart. For example, if you tried to state how "old" the Milky Way is relative to the Big Bang, you'd need to calculate our lookback time from here, and if you tried the same for M31 you'd have to go there (and then your clock would be more than 2.3 million years different).
However, we do state that the universe is largely homogeneous at large scales (much larger than the distance between Earth and M31) and that it has been expanding since the BB. So, you might reasonably suppose that if the expansion were to have proceeded at the same rate for both locations, then they're roughly the same distance from "ground zero" (wherever that may be). These are, of course, only assumptions (there is no way to measure them directly).
What we do, instead, is to state that everything we can see was at one time much nearer to everything else we can see. The only way to do that is to assume a universal expansion from some original point (ground zero). When cosmologists do that, they further assume that the smoothness of the cosmic background radiation indicates that at one time all the matter created by the BB was so close together it could "communicate" (meaning that heat and electromagnetic radiation could reach throughout the universe) and that inflation and expansion have resulted in the universe we see around us today. You could say, then, that there was a time t=0 plus some infinitesimal increment when any "clocks" would have been synchronized.
If you Google the term "time dilation" you'll find that we need not look so far afield to find differences. Moving clocks (especially those moving at high speeds relative to one another, or traveling long distances relative to one another) experience a continuous and calculable shift in time. For example, the clocks aboard GPS satellites in high Earth orbit incorporate software that accounts for the difference in time due to their distance and speed relative to ground-based clocks. In the same way, clocks aboard far-ranging spacecraft such as the recent Cassini and New Horizons missions also are compensated for time dilation effects (to ensure accuracy in their trajectories and course adjustments).