GAL 3:17 And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect. 18 For if the inheritance be of the law, it is no more of promise: butGod gave it to Abraham by promise.

This promise, or covenant, had been given to Abraham earlier, but it was only when he was faithful in his willingness to offer Isaac to God that it was confirmed. Here, the Lord tells that it was at this timethat the promise or covenant was ratified:

GEN 22:16 And said, By myself have I sworn, saith the LORD, for because thou hast done this thing, and hast not withheld thy son, thine only son: 17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore; and thy seed shall possess the gate of his enemies; 18 And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice.

And at this time, when the covenant was ratified, the prophecy of 400 years, given in Gen 15:13, begins:

GEN 15:13 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;.

This 400 years is included in the 430 years of the next passage:

EXO 12:40 Now the sojourning of the children of Israel, who dwelt in Egypt, was four hundred and thirty years. 41 And it came to pass at the end of the four hundred and thirty years, even the selfsame day it came to pass, that all the hosts of the LORD went out from the land of Egypt.

However, be sure and note that the Scriptures do not say that they will be held in bondage in Egypt for 430 years- it says that their “sojourning” was for 430 years. From the time the covenant was ratified by Abraham’s obedience in being willing to offer his only son, until the Exodus was exactly 430 years. The earlier passage said that for 400 years they would:

1- be strangers in a land not theirs,

2- that they would be subject to, or serve, the nations in which they dwelled, and finally,

3- they would be afflicted.

They were in fact only in Egypt from 1706 BC until 1446 BC, or 260 yrs.