this is quite the question. a theory in physics, relating to the ever popular "big bang", is the "big crunch" theory. basically, the big bang happened, all of the matter contained in that microscopic incredibly dense particle exploded and began moving in all different directions, thus creating the universe as we know it. the "big crunch" theory says that when all of the matter in the universe becomes too spread out, it will all fall back into the center of the universe, using so much force that all of the matter in the universe will become an incredibly dense microscopic particle. later, this particle will explode (big bang) and even later, it will form again (big crunch). if this theory is true, there has been an infinite number of universes created, meaning the earth has been made before (if the universe was created infiniti times, the solar system must have been created exactly infiniti times as well). and against all of the incredible odds (because this has happened an infinite number of times), we have all had this conversation in the past. when did time start? my answer is, time starts whenever the universe is first created, so in about 24 billion years time will start for the next universe. of course, this is only a theory, and has been the subject of debate for quite some time.