No, that is just one (entirely unproved) theory (a la Hawking). We don't know if time started with the big bang or it didn't.
The point is that IF there really was a Big Bang, there is no need to talk about a "before".
As soon as somebody asks the question, "what happened before the Big Bang," they are necessarily asking that question within the context of the model. They often expect that, within that context, there is no good answer to that question, but in fact, there is one.