A day to him is 1000 years. He created it in six thousand, then there was the Edenic period (we don't know how long that was), then Adam fell four thousand years ago. That's how it works
It's found in LDS doctrine. Abraham 3:4 talks about God's time (even though he really doesn't need it), the rest is more speculation pieced together from Latter-day revelation. And I actually made a mistake: Adam fell 6000 years ago, or in 4000 B.C.