Since according to Genesis, God didn't create the sun until the 4th day, how was a 24 hour day measured the first three days? (Hint: in Hebrew, the original language of the Old Testament, the word "day" can refer to an undetermined period of time, an era. The same is true of English, of course-- remember the movie "The Day of the Jackal"? Which day was that?)
If you're really interested in the issue, you should sit down and read the first few chapters of Genesis. And, remember, the orthodox Christian view is that the bible is God's revelation to us-- that it is true information, but not necessarily exhaustive.
Hint: in Hebrew, the original language of the Old Testament, the word "day" can refer to an undetermined period of time