Biblical chronology question

I received a question recently about a detail in the timeline for Story of the Ancient World. It concerns Exodus 12:40, which says something like this in most modern versions:

"The time that the people of Israel lived in Egypt was 430 years." (English Standard Version)

But in the Ancient World timeline, the number of years from Jacob's arrival in Egypt until the Exodus is 215 years. Why is that?

The short answer is, Ancient World follows Archbishop Ussher's chronology in all dating matters, and that is how he had it in Annals of the World. The long answer, however, is interesting. Let's look at Exodus 12:40 again, only in the Authorized (King James) Version:

"Now the sojourning of the children of Israel, who dwelt in Egypt, was four hundred and thirty years."

This translation, which more closely resembles the Hebrew, contains subtle differences. "Who dwelt in Egypt" is an adjectival phrase describing "children," and for the purposes of finding the basic grammatical meaning of the sentence contained in the subject - verb - direct object, we can set it aside for now.

Given that, we read: "Now the sojourning of the children of Israel was four hundred and thirty years."

This sojourning is mentioned elsewhere in Scripture:

"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;" Genesis 15:13

"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." Galatians 3:17

Paul said that the Law, which was given just months after the Exodus, was given 430 years AFTER the covenant. What covenant? There was no covenant when Jacob went down to Egypt. The covenant was made with Abraham. It was 430 years from the time that the Lord made a covenant with Abraham until the giving of the Law, according to Paul. So how to reconcile the apparent contradictions in Genesis 15:13 and Exodus 12:40?

Exodus 12:40 is easier; the basic sense of the sentence reads: "Now the sojourning of the children of Israel was four hundred and thirty years." As Abram was a sojourner in a land that was not his from the day he entered the Promised Land, this makes sense. The sojourning began with Abram and ended with his descendants, the children of Israel, at the Exodus, 430 years later. The phrase, "who dwelt in Egypt" is telling where the children of Israel were at the time of the Exodus. In the Hebrew, the chronology information in the sentence is not intended to go with the side phrase about the dwelling, but with the main idea of the whole time of sojourning. Sloppy modern translations have made this subtle distinction in Hebrew unclear.

Now Genesis 15:13. The Hebrew poetical construction of the sentence looks something like this:

1 thy seed shall be a stranger in a land that is not theirs
    2 and shall serve them
    2 and they shall afflict them
1 four hundred years

In other words, the four hundred years part of the sentence goes with the thy seed part of the sentence, just as in Exodus 12, thus:

"thy seed shall be a stranger in a land that is not theirs four hundred years;"

The service and affliction are again descriptions unrelated to the chronology detail.

Why 400 years here and not 430 as elsewhere in Scripture? Was God just being imprecise? No. It was 430 years from the time Abram entered into the covenant with the Lord in the Promised Land, but 400 years from the time that Isaac became “thy seed” at his weaning, which transpired 30 years later.

None of this analysis is original with me; Dr. Floyd Nolan Jones, author of Chronology of the Old Testament, explains this chronological "thorn" in Scripture with much more detail than I have. I would suggest anyone really interested in this topic to study Dr. Jones’ extensive research on it, which should settle all questions.