If you regard Genesis as literal, rather than allegorical, how would you define the terms "day", "night", "evening", and "morning" prior to the creation of the Sun on the third "day"?
Genesis 1:4b,5 ...God divided the light from the darkness. 5God called the light Day, and the darkness He called Night. So the evening and the morning were the first day.Even before He created the sun and the other lights in the heavens, He divided the light from the darkness, He called the light "day" and the darkness "night," and He ordained evening and morning. Awesome.
His audience knows the period of time called a day is 24 hours, the period of one rotation of the earth. It's been that way from the beginning.