How can we interpret "Day One" before God created Sun and Moon?
Agreed on the thousand years to one day point. These authors, under the inspiration of the One Author are pointing to God's eternal quality. I think though that it might more suggest that what we experience in time, God experiences as now; "I Am" is his name he told Moses; God, "the same yesterday, today and forever" (Heb. 13:8). A quote that sticks with me about this is: "God doesn't know last Tuesday". The meaning of this is that God doesn't relate to us the same way we relate to each other: "Last Tuesday I did this, that and the other thing", or as God might exclaim, "Last Tuesday was a great day to do the whole pillar of fire thing". God lives the entirety of time now, he doesn't set his alarm for next week.
As to the question, the "days" listed in creation are representative of time passing in the material creation process; prior to "day one" time and matter did not exist. In the cosmic order following the theory of the big bang, which is a pretty well built theory, the beginning of the created order would have taken place moving outward at at least the speed of light as we measure it today from a single burst of hydrogen. According to the theory of relativity, if we were to do an experiment where we sent an astronaut out into space traveling near the speed of light, time for him would slow to a snail's pace compared to here on earth, so that upon returning years later for us, the astronaut would have only aged minutes. This same principle shows that if we were to set a clock on the earth, the sun and the moon, time would pass by at slightly different rates. This can help to shed light on the six days of creation, the "days of old" that precede the "generations" of man (Dt. 32:7). Applying that same principle, as the universe grows and expands; as time slows to our understanding, what at that rate of speed was merely days could very well have been billions of years as we understand time today. Within that time galaxies formed and somewhere around the fourth day (extrapolating out the rate of expansion from the initial "bang") our solar system might have been formed, setting the sun and planets in motion as we see them today.
This makes it clear that the world was created in 6 days. Days as we know them. There is plenty of evidence for creation as opposed to evolution.
clarification: ....that the world was not created in 6 days......
