If God had intended for us to understand that the days of creation were long periods of time, there are several other Hebrew words used in the time of Moses which would have expressed it very clearly. The Hebrew language has 13 words indicating time, and 11 of these refer to a long period of time. For example, olam is commonly translated as "perpetual", "of old" or "forever" .

If we were trying to say that the days of creation were, in fact, literal days; and we wanted to select the most precise words to use; the best words are those which were chosen!