Actually....the author ought to be careful about that "obviously" there. Time is not constant everywhere, as any physicist will tell you, and time passes at different rates at different velocities and locations in the universe.
I've just read a book "Genesis and the Big Bang" by a Jewish physicist with an interesting theory that bridges the gap. The universe looks 15.5 billion years old if you measure from earth....BUT if you measure from the beginning at the very source of the big bang...it comes out to ...fancy that...six days. I'm not a physicist I can't get into all the nitty gritty...but check it out here on Dr. Gerald Schroeder's website. He also does a nice job of quoting supporting Scripture such as "the day of the Lord is like a thousand years", which seems to be a pretty direct contradiction to what our author is arguing here. He also quotes medieval Talmudic commentary by Maimonides, Nachmanides etc., to show this interpretation is not new-fangled but was always present in the text.
I highly recommend Schroeder's books to all interested...and I've failed to be impressed by a whole lot of these science-meets-Genesis type works.