What is a "day"?
According to modern man, a "day" is 24 hours which is the time it takes the Earth to rotate on it's axis (more or less).
According to Genesis, " God called the light 'day'. "
Genesis does not resort to any time frame accepted by modern man to define "day". It uses God's definitition that had absolutely nothing to do with the Earth's rotation around it's axis as "sky" was not created until the second day.
If you are going to takes "God's word" in interpreting the Bible, then you you need to take "God's word" that God defined "day" as "light" and not as modern man defines "day".
***Six days.
What is a "day"?***
A day is an evening and a morning, as in...
"And the evening and the morning were the first day."
-Gen 1 :5
(I would imagine that was put there so we wouldn't miss the literal nature of the "day" refered to!)
A direct correlation written in God's own hand, to eliminate any confusion.