Brutha Usser merely took the Scriptures at face value. He counted the number of generations as listed in the Scriptures, and arrived at a number. Matthew’s gospel gives a straight line list of all the generations from Adam to Christ. WE can tack on some numbers to bring it up to today. It’s not an exact science. That is why it’s stated in a range of from approx. 6,000 to 10,000 yrs. And if that is not precisely correct, I think it’ll be much closer to the truth than 450 billion is. Blessings on your day.
There is one very important part of the scripture that Young Earth Creationists seem to gloss over. Gen: 4-15,16,17. ...”Then the Lord put a mark on Cain so that no one who found him would kill him.” “So Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden.” “Cain lay with his wife, and she became pregnant and gave birth to Enoch.”
So here we see that although Adam and Eve and their two son where supposed to be the first people, there were obviously other people who were not born in God’s paradise who could kill Cain or marry him. Bishop Usser’s calculation was based on the “begats’ listed in the Bible, but we have no information on how long these other humans had been on earth. Thus it appears to me the Garden of Eden story was written to show how God had created a perfect human who would be totally obedient to him if they lacked knowledge and did not listen to snakes. Humans being what they are, the struggle has been how to become perfect humans again.