He still didn't define the length, in human terms, of those "days", did He?
I'm not keen on majoring in the minors in any case. You tell me that God's "days" are 86,400 seconds long. But they St. Peter s "day" is as a 1000 years, and 1,000 years as a day. You gonna take him to task for saying that?
The thing is, unless God is a trickster and has made the universe He created "look" quite a few millenia older than 6k years, it is in fact just that. And the real downsides to beating the tub for 86,400 second creation, or a flat earth, or a global flood, or any such thing, is that: A) you can't prove it, B) it makes Christians, and Christianity, look goofy, and C) are completely irrelevant to the Christian Faith.
Our Faith is based on God having intervened directly in human history to reconcile sinful human beings to JHimself. To steal a line from William Jennings Bryan, it's about the Rock of Ages, not the Age of Rocks. How long it took, by human reckoning, for God to create the universe has no real bearing on or anything to do with the altogether unarguable facts that:
10 [Christ] was in the world, and the world was made by him, and the world knew him not.
11 He came unto his own, and his own received him not
12 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name
It ain't about how long it took God to create the universe, or whether the earth is flat or not. St. Paul said it best: For I determined not to know any thing among you, save Jesus Christ, and him crucified. The old saint understood how things work; don't clutter your message with irrelevant stuff. The message is Jesus Christ.