How exact are the dates in Les's timeline?
The dates in the timeline are approximate, not set in concrete. Over thousands of years, a variation of about 100 years one way or the other does not make much difference. They are based on what chronologists can ascertain, such as Adam at 4004 and the Flood around 2,400 years later.
Les explains that he likes to put the timeline on the board starting back in Genesis 1, where Adam was probably created. He stresses that all these dates are approximate and that he does not set them in concrete. After all, over thousands of years of time, it does not make that much difference whether it is 100 years one way or the other. As near as chronologists can ascertain, Adam came on the scene at 4004, and the Flood, the next big event in Genesis chapters 6 and 7, was about 2,400 years later.