'day' can be a period of time, and often is, but when it is scripture clearly indicates it. In this case it clearly says during the reconstruction of the earth 6k years ago the evening and morning was the day. To apply a day as a prolonged period of time here is to ignore the context. The context tells us it is not a period of time passing one cycle of the rising and going down of the sun and moon. In the beginning God created the heaven (singular) and the earth. There's no time in that verse except to go back to the beginning. The earth has been here a very long time.

(example) we know the Day of the Lord is a period of time because holistically scripture reveals it.