Apart from what it says in Genesis (1:9) itself: God called the light "day," and the darkness he called "night." And there was evening, and there was morning--the first day.The six creative days are not literal 24-hour days. Those particular days are lengthened from the viewpoint of Almighty God. Therefore, the six creative days are of undetermined length.
Unless you're positing a vastly slower rotation for Earth, of course.