“What kind of days there are,” writes St. Augustine, “is difficult or even impossible for us to imagine” (City of God 11.6, quoted in Bettenson 1972: 436). but we need not worry about this difficulty, because we should not read “in the beginning” temporally. We need not try to mash the seven days of creation into a sequence of twenty-four hour events. Within the literary atmosphere of Gen. 1, the seven days suggest a structural, architectural order to all things, something made according to a plan rather than the ticking of a clock. The first day is the foundation of the second, and then the third, fourth, fifth, and sixth, culminating in the seventh day, the Sabbath, which does not complete the created order-for nothing is done on the seventh day-but rather crowns it. The first day tilts toward the seventh day, just as the initial separation of light from darkness presses forward through the entire sweep of scripture toward the consummating sunrise of the new creation that “has no need of sun or moon to shine upon it, for the glory of God is its light, and its lamp is the Lamb” (Rev. 21:23).
-R.R. Reno, Genesis, 48.