The oldest section of the Bible is strongly believed to be the book of Job as opposed to Genesis or other Old Testament books. The Bible starts with the Genesis, Exodus, Leviticus, Numbers, and Deuteronomy. These books were already grouped together thousands of years ago in Jewish tradition, and are known as the Pentateuch or Torah. When scriptures were considered for inclusion as part of the final Christian Bible Canon these five books in order were a logical choice to start, as the creation of man and history of Israel are very important concepts in Christianity. 

However, Job is still likely to be the oldest booked written.

The life of Job and writing of the book appears to be set between the time of Noah and Abraham, and many scriptures in the book of Job make reference to things associated with the Patriarchs vs. Israel. Job was offering individual sacrifices for himself and his family just as Noah, Abraham, and Isaac did (Job 1:5; 42:7-9). Job was getting direction from God, a relationship based on his faith same as Noah, Abraham, Isaac, and Jacob (Job 22:22, 23:12; Hebrews 11). Job is knowledgeable about the creation, Noah and the flood, and the scattering of people at the towel of Babel (Job 12:14-15; 22:15-17; 26:10; 38:8-11; Job 12: 20-25).  However, Job does not refer to Israel, the fathers of and promise to Israel (Genesis 22:17; 26:24) or the Law given to Moses. Finally, Job lived to an extended age as most did before and shortly after the flood. Job lived at least 180 years (Job 42:16). This is an estimate as he lived 140 years after his trial and recall he already has a strong house hold and grown children by the time of the trial, so 180 years is a conservative age.

Counting the genealogy:

The years between creation and Noah – Approximately 1556 years (Genesis 5). 

The years between Noah and Abraham – Approximately 400 years (Genesis 11:10-26)

The years between Abraham to Christ – Approximately 55 Generations (Luke 3), estimated to be about 2,000 years.

Taking all these ages and generations into consideration, we approximate the account of Job was taken from about 2,000 BC to 2,400 BC