The Watchtower needs to add 20 years to the beginning of his reign, or else all of the other dates in the babylonian lineage don't match. They do this arbitrarily and without any support.
WHen it comes to this 607 bc issue, the Watchtower's entire support revolves around fixating on the 70 years of servitude mentioned in the bible, and then counting that back from 539 BC when Babylon was "destroyed". Meanwhile they ignore scores of evidence from multiple sources which corroborate each other, confirming that the destruction of Jerusalem could never have happen in 607BC. It's really physically impossible.
That specifically was the straw that finally made me shut the door on this cult, because it's evident to me that the leadership clearly knows that they are wrong on this, but are too embarassed or too afraid to let the fallacy crumble for the sake of truth and clarity. Instead they perpetuate this lie to save face and in the process mislead millions.