Exodus 12:40

And the days of the dwelling of the sons of Israel in Mizraim were thirty weeks of years, (thirty times seven years,) which is the sum of two hundred and ten years. But the number of four hundred and thirty years (had passed away since) the Lord spake to Abraham, in the hour that He spake with him on the fifteenth of Nisan, between the divided parts, until the day that they went out of Mizraim.

NOW THE TIME THAT THE CHILDREN OF ISRAEL DWELT IN EGYPT WAS FOUR HUNDRED AND THIRTY YEARS. “From the birth of Isaac till now there were four hundred years. [We must reckon from that event, for only] from the time that Abraham had a child [from Sarah] could the prophecy, that thy seed shall be a stranger in a land that is not theirs,. be fulfilled. And there had been thirty years since that decree made at the ‘covenant between the parts’, Verse 18. until the birth of Isaac. And when you will reckon the four hundred years from the birth of Isaac, you will find that from the time they came into Egypt, until the time they left, it was two hundred and ten years.” Thus the language of Rashi, and it is also the opinion of our Rabbis.. This means [that the four-hundred year period will begin] from the time seed will be seen by you.”
The explanation, however, is not correct in every detail. It is written, And Abram was seventy and five years old when he departed out of Haran, and the event of the “covenant between the parts”, Verse 18. took place a long time after that. We must therefore explain the case satisfactorily in accordance with what we have been taught in the Seder Olam: “Our father Abraham was seventy years old when G-d spoke to him at the ‘covenant between the parts,’ as it is said, And it came to pass at the end of four hundred and thirty years … that all the hosts of the Eternal went out from the land of Egypt. Then he returned to Haran and stayed there five years, as it is said, And Abram was seventy and five years old when he departed out of Haran.” The sense of the verse then is to state that when Abraham finally left Haran, his native land, never to return again to his father’s house, he was seventy-five years old.
In line with the plain meaning of Scripture, it is my opinion that G-d said to Abraham, “Know of a surety that before I give you this land, thy seed shall be a stranger in a land that is not theirs for a long time — four hundred years.” He did not care to mention the additional thirty years to him [i.e., Abraham], because He told him further on, And in the fourth generation they shall come back hither, thereby informing him that they will not come back immediately at the end of four hundred years until the fourth generation when the sin of the Amorite will be full. Thus He alluded to these thirty years, for the Israelites’ staying in the desert for forty …

ומושב בני ישראל אשר ישבו במצרים (עד סוף) שלושים שנה וארבע מאות שנה, counting from the time when G’d spoke to Avraham at the covenant between the pieces. At that time Avraham was 70 years of age (according to Seder Olam). On the other hand, the “400 years” commenced when Avraham had fathered Yitzchok at 100 years of age. I have explained all this in my commentary on the covenant of the pieces.

אשר ישבו במצרים שלושים שנה וארבע מאות שנה, the sum of 430 is arrived at by commencing the count from the time G’d took Avraham out of Ur Casdim in order to conclude the covenant of the pieces with him. During that conversation with Avraham, G’d had specifically taken credit for taking Avraham out of Ur Casdim, (generally understood as saving him from the fire of Nimrod’s furnace when Avraham had been a voluntary martyr for his belief in the G’d of heaven. Genesis 15,7) This is the reason why the author of Seder Olam, an ancient historical text, describes Avraham as having been 70 years old at that time.