The central problem with the argument is that it is based on the book of genesis being historical truth, which it's not. I take a theology class at school and the book of genesis was a way for the Jewish tribes to explain something that they couldn't comprehend, i.e why is everything here? We now know more about physics, the universe etc to make much of the writing in the old testament null in terms of history. So basing his calculations on this makes his argument bullshit.
Also that quote from Mark's gospel: some biblical fundamentalist he is. Also Biblical fundamentalism makes you an idiot, there's good stuff in the bible, just take it with a grain of salt, as most of it was written for a purpose, not as an exact recount of history