St Augustine on the days of Creation

In his City of God, St Augustine said in reference to the first three days of creation:

"What kind of days these were it is extremely difficult, or perhaps impossible for us to conceive, and how much more to say!" (See Chapter 6.)

So a literal 24 hour day seemed not the plain meaning according to St Augustine. He does not venture to speculate but leaves the matter open. That was long long before modern science.