I agree with you, but *so far* we have no way of knowing what, if anything, was before the big bang. So we arbitrarily place time t=0 at the big bang. I understand your argument, but space-time itself was created at the big bang. Space-time = our 3 spacial dimensions and our 1 time dimension. Therefore, it follows that time was created in the big bang (so there is not time b4 the big bang, at least no time that we can measure). Of course, if Andrei Linde's 2nd Inflation Theory is correct, other universes could have been there before (before implying time) our big bang, but how could we measure when if space-time for our universe was created in our big bang?
Ah, well, I've succeeded in confusing myself here. I guess I'd better stop before I go cross-eyed ;)...