Well, I'm sure there are geological reasons as well.
My point is that there's no reason scientific theories that put the age of this planet at billions of years has to contradict what the Bible says. This is putting evolution aside (and speaking strictly on geological terms).
Well, as I understand it, the Hebrew word for "day" meant a 24 hour day. Also the day and night designations did not need the presence of a sun, as there was light before the sun. Probably not from stars though, as they seem to have been created on day 4.