The Watchtower has to change the entire chronological dating because of its perculiar insistence that Jerusalem was destroyed by Nebuchadnezzar in 607 BCE. Nearly all historians date his rule from 605 - 562 BCE. Only the Watchtower stands alone against the tide of facts.
As has been noted by a number of scholars, the "70 years of desolation" spoken of in Jeremiah applies not to Jerusalem alone but to the other nations surrounding Judah as well. (Jeremiah 25:9 - 11, 15) Babylon started its invasions of the "other nations" well before its destruction of Jerusalem. (586 BCE) So, it would seem logical that the 70 years would have been inclusive of all these events.