Barbour used what he considered to be the best dates for two events that became key in Russell's chronology in order to arrive at 606 B.C. for the destruction of Jerusalem: 536 B.C. for the fall of Babylon, and 536 B.C. for the return of the Jews to Judah. Following the lead of certain earlier prophetic speculators, he just added the 70 years spoken of by Jeremiah to 536 and got 606. Of course, this was completely wrong.
We now know that the above-mentioned events happened in 539 and 538 B.C. respectively. While many historical commentators were convinced that the dates Barbour used were correct, a good many others used other dates, including the ones accepted today.
It's pretty amazing that Barbour and Russell managed to misunderstand something as fundamental to historical dating as the lack of a "zero year" between 1 B.C. and 1 A.D. It's even more amazing that JWs today don't see this as a serious blow to the Watchtower Society's longstanding claim that Russell was "divinely directed" in presenting his chronology.
About 1904, Russell seems to have, for the first time, seriously taken note of the zero year problem. He changed a number of his expectations regarding 1914, so that by about 1910 he had speculated that 1915 might be the "correct" year for the end of the Gentile times. Around that time, one of his lieutenants, P. S. L. Johnson (the one who Rutherford kicked out of Bethel in 1917), pointed out that, to retain 1914 as the magic date, the date for Jerusalem's destruction should be moved back to 607 B.C. However, Russell never did anything with this information. That this was known to studious Bible Students is easily proved: Morton Edgar, in his 2nd volume of Great Pyramid Passages (1913), listed 607 as the date of the destruction; the 1917 WTS book The Finished Mystery also listed the 607 date. Rutherford, apparently not wanting to upset the apple cart, did nothing about any of these problems.
It was in 1943 that Fred Franz finally began to deal with many of the chronological problems left over from Russell's time. It took him another dozen years to sort things out to the state the Watchtower chronology is in today. Many dates were changed: 606 went to 607; the date of 536 for Babylon's fall went to 538 and then 539; the date of 536 for the return of the Jews went to 537.
In the middle of the 1943 book The Truth Shall Make You Free, Franz changed the date for the start of the Gentile times from 606 to 607 B.C. He did this by telling a lie: he changed the date by exactly one year, but implied in his explanation that the change was only a few months. However, he left the date of Jerusalem's destruction at 606 B.C. This presented a problem: according to Franz, the Gentile times began about 10 months before Jerusalem was destroyed. This was "fixed" in the next year's book, The Kingdom Is At Hand. It presented a chronological chart showing -- without any explanation -- Jerusalem's destruction as being in 607 B.C. Franz included a footnote that told another flat-out lie: the footnote claimed that the reason for the date change was to be found in the 1943 book. But there was no such explanation in the book, and it's easily shown that the last mentions of Jerusalem's destruction in the book pegged it as 606 B.C.
For a lengthy and detailed look at this whole matter, see the article "The Evolution of 606 to 607 B.C.E. in Watchtower Chronology" at this link: http://www.geocities.com/osarsif/606.htm . There are several other shorter articles with obvious titles at that website.
AlanF