Genesis 6—9 records the events of Noah’s flood, also called the Great Flood. If the genealogy provided in Genesis 5 is intended to be comprehensive, we can determine the dates of various events by simply adding up the time spans between fathers and sons, given in Genesis 5:
Adam to Seth — 130 years
Seth to Enosh — 105 years
Enosh to Kenan — 90 years
Kenan to Mahalalel — 70 years
Mahalalel to Jared — 65 years
Jared to Enoch — 165 years
Enoch to Methuselah — 65 years
Methuselah to Lamech —187 years
Lamech to Noah —182 years
According to this method, the time from Adam to Noah was approximately 1,059 years. These are approximate times because we don’t know if the years are counted from conception or birth; also, it is obvious that the years are given in whole numbers but no doubt included (or excluded) partial years. (For instance, Adam was 130 years old when Seth was born, give or take a certain number of months and days.)
So we have Noah’s birth, which occurred about 1,059 years after the creation of Adam. Then, in Genesis 7:11, we are told that the flood came in the 600th year of Noah’s life, so that would mean the Great Flood came approximately 1,659 years after Adam was created in Eden. Using those same numbers places the creation of Adam and Eve at around 4,004 BC. So, doing the math, Noah’s flood occurred in approximately 2,345 BC.
Similar genealogies are found throughout the Old Testament. Using the same method places Abraham at 228 years after Noah’s flood or about 1,887 years after the dawn of humanity. We can also use the genealogies to count backward from other dates that we know, such as the fall of Jerusalem. Using this method, Abraham would have come on the scene about 2166 BC, and the exodus during Moses’ time would have happened about 1446 BC.
Some scholars believe that the genealogies are not intended, and were never understood by the original audience, to be exhaustive. It could be that generations were skipped, as we know happened in the genealogy of Jesus recorded in Matthew. If this is the case in the Genesis 5 genealogy, and there are years unaccounted for, then we really have no idea when the Great Flood took place.
The actual year of the flood is less important than the meaning of the flood, which is that God punishes sin but also provides a way of salvation.