The Gentile Times Reconsidered is not a great book because he does not reconsider anything. He just tries to prove that 587 is the destruction of Jerusalem. But if he was reconsidering the Gentile Times, he would know that the Gentile times did not start at the destruction of Jerusalem, though that was taught by the organization since Russell. He, like the WTBTS, can't see the forest through the trees.
There are several ways to confirm 607B.C; unfortunately it seems that only one has bothered to research without just accepting things that have been mentioned as fact when it lacks foundation. But as someone stated it is a long drawn adventure. By the way time table goes backwards. So the essence of the American civil war is part of the timeline.