Now when the sun was going down, a deep sleep fell upon Abram; and behold, terror and great darkness fell upon him. Then God said to Abram, “Know for certain that your descendants will be strangers in a land that is not theirs, where they will be enslaved and oppressed for four hundred years. But I will also judge the nation whom they will serve, and afterward they will come out with many possessions.
Genesis 15:12-14 (NASB)
Genesis 15:12-14 echoes Genesis 1:1-3.
The number 400 is represented by the Hebrew letter "tav," which is the final letter in the Hebrew alphabet. It points to a finality; perhaps captivity in Egypt is a picture of something larger.
It is the past. It is the future. It is now.