Genesis 1:14 tells us "Then God said, “Let there be lights in the firmament of the heavens to divide the day from the night; and let them be for signs and seasons, and for days and years". Here, He describes the creation of the sun and the moon, and more importantly their purpose. In effect, the sun is a giant clock. It is an indicator of time, dividing the day from the night.
This basic concept is a bane to a very common argument against Genesis. If the sun wasn't created until day 4, how can days 1-3 have occurred? This question, according to Genesis 1:14, is akin to asking how an hour existed before the pendulum clock was invented in 1656. As an hour does not need a clock to pass, a day does not need the sun. Rather, the sun tells us when a day has passed.
Some respond by pointing out that different planets have different days. Pluto, for example, has a day equivalent to nearly 6 and a half earth days. A simple question for those people: How many people are reading the Bible on Pluto? Without getting into silly debates about the possibility of aliens, or speculation about future space travel and colonisation, at this point in time, 100% of the human race has always lived on the earth. A very small minority have ever been into space, and even they came from the Earth. What sense would there be in God saying "for in 6 days (or in 1 Pluto day, or 11 Saturn days etc.) the Lord created the heavens, the earth, and all that is in them"?
I'll tell you what, let's do an experiment. Go to any store in your local area, pick out something you like, look at the price tag, and ask the staff if the price tag is accurate. Let's say you're an American, and the product is $20. When the staff member informs you that yes, the item is $20, point out that this is equivalent to just under £16 in England, then see exactly how much they are willing to negotiate the price. My guess is you will eventually be asked to leave the store, without the item.
The argument is silly. The fact that the sun was created on the 4th day has no impact either on the consistency, nor the historical value of the Genesis account of creation. When God said He created in just 6 days, He created in just 6 days. We need not play around with this.