The simple answer is if there is a God who is a creator then he exists out with the boundaries of the physical universe and with that time...we humans cannot grasp the concept of outwith time...the bible says that God is the "King of Eternity" and thus He will live forever...thus put that concept backwards and he has always lived or been in existence...something we cannot grasp because we are limited to the concept of time.
You can't understand a God in terms of human boundaries...