Since Darwin, there have been Christians who attempted to interpret each day as an age. I think this came mainly from the pressure of wanting to be accepted by the world of science. The best reason for interpreting Creation as literal days is that the people (Rabbi's) who know and study the original text best all agree they are literal days. The Talmud was complete in 5 A.D., which was the code of Jewish oral tradition interpreting the Torah or first 5 books of the Old Testament. If there would have been any hint that these words referred to an 'age', these meticulous people would have picked up on it.