You call it fable and that's cool. We have ledgers and notes but after a few thousands of years back all we have is speculation, hhypothesis and fables as you mention. Science or spirituality both require a bit of faith (or belief). But to your point, if God was creating the laws and gravity and time all the things that make the earth work as we know it why would a day then be the same as we know it now? I fail to see how it (a day) being different negates the use of the fable as you put it. Genesis wasn't written at the time of creation. So perhaps if when God gave it to Moses He had to do it in away that he could comprehend. If you explain complex topics to a child(or novice) do you not break it down so that they can understand and give them more as they grow?