I have been asked this question before. My answer is similar to the authors.
We, here on earth measure time in relation to the rotation of the earth about its axis and its orbit around the son. So in that measure 4.5 billion years. However, as God is not of this earth it would be silly to assume that he would measure time by the same standards. So in regard to your real question, which apparently are afraid to ask, do I believe that God created the universe? Most definitely. After all, what is the measure of a day in the life of an eternal being?
I have been called derogatory names for this answer, and told that it is a stupid answer, but I have never had a lib be able to either answer that question.