What we call "proofs" the earth really have billions of years is not a so strong proof.

For example, lets imagine someone goes to a theater, but arrives late, so the person watches the movie from the middle, instead from the beggining. Im supposing the person is alone in the theater (i know its sad, its just for the argument's sake). My question is, how this person can be absolutely sure the movie really started from the beginning? It could have started, from the middle, only moments before this person arrive to the theater. So, this person just could not tell the difference.

The same happens with us. We are here upon the earth only some millions of years (at most). How can us be so sure of things that happened much before we're here? How can anybody prove, in an unquestionable way, that the world was not created just a little time before?

Once, one friend of mine that was given to this phylosophies, and who showed me this argument, went even further: we cannot prove that even yesterday really happened. The universe could have been created today, and we could have been created today too, but our minds filled with memories to simulate that we were here before, and that we had a past.

I struggled a lot against him, but in the end i had to agree. Really we cant prove or disprove it. Since that day i stopped to take so much things for granted. We think we know much more than we really know. But now im aware of it.