Neither day-age, nor Augustine’s simultaneous creation is to be found anywhere in the Bible. Genesis is very simple and straightforward, so that even a child can understand it. Creation lasted 6 days, and on the seventh day God rested. Notice, I don’t have to add anything to the Bible, nor take anything away to say that.
A child can understand that pregnant mommy has a baby in the belly. That doesn't mean, though, that he comprehends a fetus in the amnion implanted on the wall of the uterus.
Likewise, yes, the earth was created in six days. Period. End of story. I believe that. But--as they knew to ask in even ancient times--what kind of days are these without the sun to mark them? Some claim they are literal 24 hour periods. Fine. That's one interpretation. But there are others. And we also know from general relativity that "24 hours" isn't even a fixed value across the universe...it proceeds at different rates in different places. So I'm not sure why people are so dogged on this point.
And actually, Augustine got his simultaneous creation idea DIRECTLY from the Bible. The Vetus Itala version of it, to be specific. It's commonly believed that he based his interpretation on a Latin mistranslation...which ought to teach us a further lesson about relying too heavily on English texts to the exclusion of the Hebrew, Septuagint, Peshitta, and Vulgate readings.
Is that Saturday or Sunday?
Which biblical version of the creation of woman is correct? The version where God created man and woman from the dust, or the version where God took a rib out of Adam and made a woman from it. Both there, both with very different implications. One egalitarian man to woman, the other with woman subordinate to man. The bible is full of contradictions which enables many different interpretations to be made. This also causes problems with the Koran, much to our current distress.