well, maybe God made the earth, already a few billion years old... what I mean is, that when he made adam, I don't think he made him as a newborn baby, but that when he was created that adam was already a grown man... so God could have made the earth pre-aged (much in the same way you could buy a Gibson Les Paul special faded, where it already has a faded finish when you buy it brand-new...) that is one possible explanation I think.