KJV
Genesis 15

1:   After these things the word of the LORD came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward.

2:   And Abram said, Lord GOD, what wilt thou give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus?
3:   And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir.

4:   And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
5:   And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.

6:   And he believed in the LORD; and he counted it to him for righteousness.

7:   And he said unto him, I am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it.

8:   And he said, Lord GOD, whereby shall I know that I shall inherit it?

9:   And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon.
10:   And he took unto him all these, and divided them in the midst, and laid each piece one against another: but the birds divided he not.
11:   And when the fowls came down upon the carcases, Abram drove them away.

12:   And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.
13:   And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, (and shall serve them; and they shall afflict them) four hundred years;
14:   And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.
15:   And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.
16:   But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.

17:   And it came to pass, that, when the sun went down, and it was dark, behold a smoking furnace, and a burning lamp that passed between those pieces.
18:   In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
19:   The Kenites, and the Kenizzites, and the Kadmonites,
20:   And the Hittites, and the Perizzites, and the Rephaims,
21:   And the Amorites, and the Canaanites, and the Girgashites, and the Jebusites.

1. v.1-6 The timing of this event: The beginning of the 14th of Abib (Nisan), the timing of the Lord's Passover.
Days begin at sunset.

4. This shall not be thine heir...shall be thine heir. 1st Promise of God towards Abram: Physical Seed (Isaac)

bowels = loins.

5. Look now toward heaven, and tell the stars... This event took place at night (14th of Abib (Nisan)), when there was no sunlight, so as to see all of the stars of heaven.

tell = count.
tell the stars...So shall thy seed be. 2nd Promise of God towards Abram: Spiritual Seed.
See Mt. 13:43; Da. 12:1-3.

7. v.7-11 The timing of this event: The day portion of the 14th of Abib (Nisan).

9. v.9-21 The animals of the sacrifice were also offered at the future Temple. Notice that here there is no alter and no splashing of the blood, though it is a very bloody affair. The bloody sacrifice of the animals sealed this special covenant between God and Abram, of which was a maledictory oath in which God pronounced His own death to fulfill it.

10. divided them in the midst = divided them in the middle, i.e. cut it down the middle from head to toe.

laid each piece one against another Placed back to back, creating a path between the parts.

12. v.12-16 The timing of this event: The day portion of the 14th of Abib (Nisan) in the late afternoon, when the sun was approaching the horizon.

a deep sleep fell upon Abram Abram was to have no part in this covenant oath. The only responsible party now is upon God to fulfill it (Ga. 3:20).
Cp. v.17
Cont. Ge. 21:27.

an horror of great darkness fell upon him While in a deep sleep (and obviously in a vision), God had Abram experience death (though without dying).

14. v.14 Prophecy of God's promise to deliver the children of Israel from Egypt, even to leave with great wealth.

17. v.17-21 The timing of this event: The beginning of the 15th of Abib (Nisan), the timing of the Lord's 1st Day of Unleavened Bread and the future timing of the exodus of the children of Israel.
Days begin at sunset.

a smoking furnace

"Symbolic of the affliction of Israel (Deut. 4:20; I Kings 8:51; Isa. 31:9; Ezek. 22:18-22; Jer. 11:4)"

-- E. W. Bullinger.

a burning lamp

"Symbolic of Israel's deliverance (I Kings 11:36; 15:4; Isa. 62:1; II Sam. 21:17)"

-- E. W. Bullinger.

13. A better translation...
"And He said to Abram, You must surely know that your seed shall be sojourners in a land that is not theirs, (and shall serve them and afflict them) four hundred years."

and shall serve...afflict them

"Figure of Speech. Epitrechon.
"A sentence, not complete in itself, thrown in as an explanatory remark. A form of Parenthesis."

-- The Companion Bible
Appendix 6

This portion must be rendered in parenthesis to put what was said in context. Nearly all translations render this verse to indicate that Abram's descendants would be in slavery for 400 yrs. total, but in truth Scripture reveals that their enslavement was probably around 150 yrs.