Basically i think what Arachophilia is point out is that you can date Genesis to aproximately 900 B.C. due to place errors mentioned in the text.
An example of this would be any history textbook that labeled San Jose as the current capital of the state of California. Since San Jose was the capital 1850-1854 the history book would then have to be written at latest in 1854.
Similiarly i think genesis would be dated to 900 B.C. because of the name of cities that were present at that time frame.