Ussher Chronology, Bischop Ussher added up the different dates given for the ages and Birthdates of the Patriarchs and the ammount of years officially given between key events in the 1600s.
In Genesis there is a list of the initial 10 or so Patriarchs for instance and how old they were when their sons were born, so you just add all those dates up.
It is not perfectly consistent, there is contradiction for instance in the following verses
Galatians 3:15-17 - The period from the calling of Abraham to the introduction of the Law (First year of the Exodus) was 430 years
Acts 7:6-7 - The Israelites were enslaved and ill-treated (in Egypt) for 400 years
Genesis 15:13 - The Israelites were enslaved and ill-treated for 400 years
Exodus 12:40 - The Israelites lived in Egypt for 430 years
If the enslavement was literally 400 years, then both Abraham and Isaac would still have been alive by the time the enslavement began if all 4 verses are true simultaneously, fundamentalists get around this with some bullshit reinterpretation where "enslavement" and "living in Egypt for 430 years" get reinterpreted as "soujouring around".
1 Kings 6:1 Says that the time from the start of the Exodus to the building of Solomon's Temple was 480 years.
So: 480 (Time from building of Temple to Beginning of Exodus/Giving of the Law, 1 Kings 6:1) + 430 (Time from beginning of Exodus/Giving of the Law to calling of Abraham) + 75 (Age Abraham was when he was called, Genesis 12:4) + 1948 (Years from the birth of Adam to the birth of Abram, derived from info in Genesis 5,11 and 12) = 2933
The Temple of Solomon, if it existed, would have been built between 1000-950BC, so 975+2933= Beginning of Creation around 3908 BC
You'll notice that I'm off by about 100 years, if you want to know why you'll have to go through the Bible and Bischop Ussher's reasoning yourself, cause I don't know either, despite trying to go through the bible and replicate his chronology myself.