I thought they (in the past anyhow, I don't know what their present truth is at the moment) fudged 607BCE for the fall of Jerusalem by saying that;
a) the Jews returned from exile in 537BCE, and
b) the 70 years mentioned in various scriptures (can't remember the exact details, one of them's in Jeremiah though) refers to the length of time that the exile lasted, from the fall of Jerusalem.
Hey presto, -537-70 = -607. I thought that was the basis of their "biblical chronology". The two things wrong with that (again, as far as my imperfect memory recalls) are that the 70 years in Jeremiah (and elsewhere) more logically refers to the duration of Babylon's rule over the region rather than the exile of the Jews, and that there's no good reason to assume that the Jews returned to Jerusalem in 537BCE.
For goodness sake don't quote me on any of this! This is only how I remember it from chapter 7 (having a quick look in my copy) of "Gentile Times". Greatful though I am to COJ for writing it, that book has to be one of the most boring books on planet earth! I'm falling asleep just thinking about it.....zzzzzzzzzzzzzzzzzzzzzzz
ig.