One of his days could be 500,000,000 of our years.
Genesis 1(14-19):
14 And God said, “Let there be lights in the vault of the sky to separate the day from the night, and let them serve as signs to mark sacred times, and days and years, 15 and let them be lights in the vault of the sky to give light on the earth.” And it was so. 16 God made two great lights—the greater light to govern the day and the lesser light to govern the night. He also made the stars. 17 God set them in the vault of the sky to give light on the earth, 18 to govern the day and the night, and to separate light from darkness. And God saw that it was good. 19 And there was evening, and there was morning—the fourth day.
I have always wondered what the definition of "night" and "day" are prior to the creation of the sun on "day" 4.
It's interesting that on day 4 God made two "lights"--it doesn't say God made two heavenly bodies.
Perhaps the sun was already made, and day existed but in a very diffused way through a perpetually overcast sky (as on Venus). Day 4 may be referring to the oxygenation/clarification of the atmosphere allowing an earthly observer to actually see the lights in the firmament.
That's one theory anyway. :)