There are some fairly good reasons to think that "day" in genesis means one 24 hour day. Firstly, the bible uses it to mean 24 hours in the vast majority of passages. In the bible, when the word day is used in conjunction with a number..ie 3 day's travel...it means 24 hours time period. In Exodus 20:11, God states that the whole of creation took 6 days. Even each day is bounded with an evening and a morning.

Now I agree that the earth is 4.3 billion years old. But in this bit of history, the bible is simply wrong. Make of it what you will...