As others have pointed out, he also said through St. Peter that for Him 1 day was as a thousand years to us, and vice versa. So what's 1 day in Genesis mean really?
The thing is, when it comes to making assumptions about the nature of God the only one I make is that I can't hope to fathom it or reason it out. I'm no Aquinas or Calvin or ... pick your personal favorite theologian ... so if others are smarter and more well read and all that, cool. I'll stick with not trying to figure out why the science we have is all wrong because it doesn't match my assumptions about God.
If you read Hebrew, you'd understand that the word used for "day" in Genesis literally means 24 hours.