I've heard it said that there were a Jewish sect that predicted that there are 3 periods of 2000 years. They predicted this before Jesus' coming, I believe. The first was without the law, the second was with the law, and the third, this one, is the one with the law completed through Christ. And then the fourth period would be one of 1000 years. This seems to be a decent theory, as it would make perfect sense, what with God's use of the number 7. 2000 + 2000 + 2000 = 6000 years of man's imperfection (6 being then number of imperfection), + 1000 years of an Earthly Kingdom established by God, would equal 7000 years (7 being the number of perfection).
As talitha said, for God a thousand years is as one day, and vice versa. I've heard it explained that God is timeless and has made time for our benefit, which makes perfect sense to me. That would explain how He doesn't change - He's in the exact same state now as He was a thousand years ago. He simultaneously makes all decisions that He's made in our world, because time does not exist for Him.
In addition, the law and the prophets were fulfilled through Christ, in that while with the law there was condemnation and no salvation, and in the first period there was no law at all, now there is both the old law, cast aside, and salvation. We don't need more revelation because God has given us all we need for this period.