The bible isn’t just one book. It is a compilation of books. I’m quite sure you’re aware of that.I don't think the Bible mentions anything about 24 hours, but it does say a morning and an evening was considered a day. Obviously, those who wrote the Bible weren't there to see creation unfold, but they wrote it in such a way so that it would make sense to those reading it such that a day was considered a day by normal standards.
Also, I doubt that folks sat around with their chisels and hammers thinking how to word stuff, so that their potential audience would comprehend it. I’m just saying.