In a sense, the "moment" of the big bang is the zero of time for this universe. Of course, this may not be the only universe, Hawkins plays with a number of models in which there are multiple universes in The Large Scale Structure of Spacetime. And, of course, the big bang may not be true. I'm still hoping that the steady state universe wins out in the end, but the evidence against it is pretty strong.
Of course, no "theory" can ever be "proved all the way down to Absolute Truth,", that's what makes them "theories". It is another of my beliefs, but I don't think we'll ever fully understand -- we'll just keep getting closer.
It is a journey without a final destination, IMO. But the trip is worth it for itself.