Hi rubiks,
Can't speak for how others determine this number, but here's how I have.
Day 1 of the creation of this realm God speaks into existence the earth.
Day 6 of the creation of this realm God forms the first man, Adam, from dirt.
The first man, Adam, lives until he is 130 years and has Seth.
Seth has Enosh when he is 105. (This realm of creation is now 135 years old.)
Enosh has Kenan when he is 90.
Kenan has Mahalalel when he is 70.
Mahalalel has Jared when he is 65. (The realm in which we live is now 360 y/o)
Jared has Enoch when he is 162.
Enoch has Methusalah when he is 65.
Methuselah has Lamech when he is 187.
Lamech has Noah when he is 182. (God's created realm is now 956 y/o)
According to the literal reading of the Scriptures, everything that exists in this realm that God created for man is 956 years old on the day that Noah is born.
Noah was at least 500 years old when he became the father of Shem, and died at the age of 950. However, the flood came when Noah was 600 years old and so we know that there can only be a 100 year discrepancy as to when Shem was born because he was with Noah in the ark. But, if we read carefully Genesis 11 we find that Shem was 100, two years after the flood. So, allowing for some fudge room, the creation is within 2 ears of 1,456 years. (he was born to Noah after Noah was 500. The flood came when Noah was 600. When Noah was 601 the flood waters were dried up) So, depending on whether one decides that 'after the flood' refers to the day it began or the day it was said to be gone, we must allow some wiggle room. Two years after the flood Shem was 100 years old.
When Shem was 100 he had Arphaxad. The creation is 1,558 years old =/- 2.
When Arphaxad was 35 he had Shelah.
When Shelah was 30 he had Eber.
When Eber was 34 he had Peleg.
When Peleg was 30 he had Reu.
When Reu was 32 he had Serug.
When Serug was 30 he had Nahor.
When Nahor was 29 he had Terah. (the creation is now 1,788 y/o +/- 2)
Sometime after Tereh lived 70 years, but before he lived 205 years, he had Abram.
However, we read that by the time Terah died Abram had married and they had begun a journey to Canaan. The family settled in Haran and Terah never made it to Canaan. Therefore, we can shave quite a few years off of that 135 year gap if we logically figure that the marriage of Abram to Sarai probably would have been several years before Terah died. The other option is that Abram, Nahor and Haran were triplets and all born when Terah was 70 years old. Either way, any gap would likely be less than probably 100 years.
So, now the creation of this realm which God made for man is now 1,988 years old +/- 60 years. I added the median number of years (140) and that should put us either up or down 60 years from 1,988 years to the birth of Abram.
Abram (now Abraham) is 100 years old when Isaac is born. (creation age 2,088 +/- 60)
Isaac was 60 when Rebecca had the twins, Jacob and Esau.
Jacob was 130 years old when he went to Egypt. (age of creation 2,118 +/- 60)
Now, there is some disagreement as to exactly how long Israel was in Egypt, but the Scriptures declare it to be 430 years. So, creation to the Exodus, according to the Scriptures, 2,548 +/- 60 years.
40 years Israel wandered in the desert and then we have to research the judges and kings accounts to determine the years, but most biblical studies put the number of years between the exodus and Jesus coming @ 1,450 years, but we do have to allow for some fudge factor there, but, at most only a couple of hundred years.
So, 2,548 years of creation to the exodus + 1,450 years to Jesus + 2014 (today's year count) puts us @ 6,012 years +/- 200 from the day that Adam was created to me sitting at this computer. The secondary support for this total comes from Israel itself. Remember that they are God's witnesses to the world. The Hebrew year, which is fairly carefully calculated from what Israel believes to be the beginning is 5,774, although there are some disagreements there also, but only again, a couple of hundred years.
I hope that this helps. God bless you.
In Christ, Ted