Anyone who starts questioning the meaning of "day" in genesis is simply a modernist. Scripture is very clear that "there was evening and morning" on the first day. Day does not mean an age, or a period, or an epoch of time. It means day. God tells us to have the simplicity and faith of children. Accept words as God created them to be understood. That's called humility. As if God couldn't create the entire universe in 1 SECOND. So He spread it out over 7 DAYS, for a reason.
If you start questioning Scripture "I mean, does a day really mean a day?" You're on the road to losing your Faith.
Sure, St Augustine was a big questioner of many things, but we only know this because he was a voluminous writer. He wrote 1 thing and then changed his mind later. He wrote a LOT. There's a reason why Modernists love St Augustine, because you can find quotes on almost any topic, for different views. He philosophized out loud, in the written word. He was not consistent.
Given all of that, St Augustine didn't believe in evolution, nor did he believe in this modern-anti-church idea that Creation happened over millions of years.