Actually the transition from 606 to 607 didn’t happen until the 40’s, post-Rutherford, and it happened by steps.
Russell's chronology (or Barbour’s) had the 70-years end with the fall of Babylon, which was actually the right event to end the 70 years with (Jeremiah 25:12), but it was dated to 536 BC. Therefore, by subtracting 70 years from 536 BC, they came to 606 BC. That is how they dated the destruction of Jerusalem.
As mentioned, the Gentile Times equation did not account for there being no 0 year. Therefore, 2520 years added to 606 BC, ended in 1915.
So the chronology of calculating 1914 from Jerusalem's destruction had two errors.
Once it became apparent that Babylon fell in 539 BC, not 536 BC, that had to be corrected. However, subtracting 70 years from that yielded 609 BC, therefore the Gentile Times equation would come out to 1912.
So the event that ended the 70 years changed from when Babylon fell, to when the Jews returned to Jerusalem and ended the Exile. This was set at 537 BC, although there is no way to prove this from Scripture. It is conjecture. Besides, Ezra 1:1 and 3:1 seem to indicate the Jews returned in the fall of 538 BC.
However, that was the only way they could get the Gentile Times equation to work (this time accounting for the 0 year).