Question: A couple of people have queried the 430 years, mentioned in Exodus 12:40. Was it from Abram's departure from the city of Ur or the time the children of Israel spent in Egypt?
Answer: The 430 years starts from when Abraham left the city of Ur, that’s when Abraham began the journey and when he received the promise (Gen 12:1-3) - and that’s the point Paul makes in Gal 3:16–18 - "The promises were spoken to Abraham... the law, which came 430 years afterward." (The law was introduced in the third month after the Exodus).
The text of Exodus remarks that the Israelites marched out on the “very same day,” - "At the end of the 430 years, to the very day, all the LORD's divisions left Egypt" (Exodus 12:41), which tells us that Abraham marked that day as special and told his descendants about it too.
When the LORD God and Abram agreed on a contract, Abram was told that his descendants would be strangers, or, as the Septuagint says, “sojourners,” in a country not their own for 400 years (Gen 15:13). Isaac’s birth was the starting point for this period. Isaac was born in Canaan but the country didn’t belong to him or to his father, in fact Abram himself was still considered a relative “newcomer” to the area—a Hebrew, one of the Mesopotamians who had “crossed over” the Euphrates.
The LORD God had told Abram that in the fourth generation his descendants would return to Canaan (Gen 15:16), which means the four generations begins with Jacob because his father, Isaac, never left Canaan, but Jacob died in Egypt.
We can see this highlighted with Moses’s family tree - The first generating was Jacob fathering Levi, the second was Levi fathering Kohath, the third was Kohath fathering Amram, and the fourth was Amram begetting Moses who led the people back towards Canaan in what is known as the exodus.
Isaac’s birth as the starting point for Yahweh’s 400-year prophecy was an important feature of Jewish history.
We know this because Paul, in Acts 13:16–20, points his fellow Israelites and some Gentiles to the 450-year period that ended when the Israelites’ conquest of the land of Canaan was complete. Paul started his calculation from when Isaac was born, which is fairly easy for us to work out.
There were 430 years from Abram’s promise to when the law was given to Moses (Gal 3:16–17).
Caleb said he was 40 years old when the first group of spies went to survey the land, which happened around 2 years after the law being given to Moses.
If we add 2 to 430 we get 432.
Caleb also states that he was 85 years old when he asked for the mountain as his part of the inheritance of the land towards the end of the conquest. That’s 45 years for Caleb, which we need to add to the 432, giving us 477 years from when the promise was first given to Abram.
Then we need to add the time it took Caleb to remove the Anakites from the mountain before the land had rest from war, which wouldn’t be too long, probably about 2 or 3 years, taking us to 480 years.
Now we need to take away 30 years, because Abram was 70 when he received his first promise and 100 when Isaac was born. So if we take 30 away from 480 we are left with Paul’s 450 years. Paul himself calls the number an approximation, but it’s precise enough to see the starting point is taken from Isaac’s birth