History of the Change of 606 to 607 B.C. in Watchtower Chronology
First a bit of history on the 606 B.C. date. In the mid-19th-century a small number of Bible writers decided to interpret the "70 years of Jeremiah" as applying from the fall of Jerusalem to the fall of Babylon. The latter was commonly but not universally held to have happened in 536 B.C. Seventy years earlier gets you to 606 B.C. By 1871 Nelson H. Barbour had adopted this idea and had begun constructing a "Bible chronology" based on it, including a prediction that Christ would return in 1873. After Christ failed to appear, he predicted 1874. After Christ again didn't show up Barbour searched for reasons why his predictions failed, and by 1875 had settled on the idea that Christ had in fact returned, but invisibly.
Enter Charles Taze Russell in early 1876. Russell read about some of Barbour's ideas in Barbour's magazine Herald of the Morning, connected up with him, and eventually adopted all of Barbour's ideas on "Bible chronology". This included the idea that 2,520 years of "Gentile rule" would end in 1914.
To make this "Gentile times" calculation Barbour and Russell started with the incorrect date of 536 B.C. for the fall of Babylon and the return of the Jews from exile, worked back to 606 B.C. via "the 70 years", added 2,520 years (from 7 "gentile times" multiplied by 360 "prophetic years" per "time") and arrived at 1914 as a significant date. Unfortunately they were poor mathematicians for forgot that there was no "zero year", namely, that from the beginning of 1 B.C. to the beginning of 1 A.D. was one year, not two. They should therefore have calculated 1915, not 1914.
About 1904 Russell realized that there was a serious problem and started hedging on 1914, sometimes writing that 1915 could be the magic date in his chronology. In early 1914 one of the Society's directors pointed out to Russell that his base dates for his chronology were incorrect in light of findings by the 1890s that Babylon had actually fallen in 539 B.C. and the Jews had returned in 537 or 538 B.C., and therefore that the 606 date needed to be moved to 607 B.C. Russell never did anything with this information, but it was published in 1917 in The Finished Mystery and in a chart of chronology in a 1935 Golden Age article. The Society stuck with the 606-based chronology until shortly after Rutherford's death in 1942.
At that point Fred Franz became the real head theologian and all-around scholar for the WTS. In 1943 he wrote the book The Truth Shall Make You Free, which was a kind of summary of much of JW teaching at the time.
Freddie used the 606 B.C. date for the fall of Jerusalem and the beginning of the Gentile times until about the middle of the book. At that point he decided to change the date for the beginning of the Gentile times to 607 B.C., which he did by explaining that the "vulgar year" actually has its start in the autumn of the previous year, so that rather than begin in 606 B.C. the Gentile times had to begin in the autumn of 607 B.C. Doesn't that make sense? See page 239 of the book for details.
Unfortunately, Freddie forgot to change the date for the fall of Jerusalem back to 607 as well. So throughout the rest of this 1943 book, Freddie wrote that the Gentile times began in the autumn of 607 B.C., and Jerusalem fell about ten months later in the summer of 606 B.C. He noted that this created a conceptual problem and so a year later, in the book The Kingdom Is At Hand, he moved the date back by one year. See page 171 for details, where you'll find a footnote that explains that the date was actually changed in the 1943 book (it wasn't).
Freddie's explanation of the change of the Gentile times date is a fine example of a snow job. Russell had always begun the Gentile times in the autumn of 606 B.C. Freddie's explanation implied that his changing the 606 date back to 607 B.C. was a matter of only a few months whereas he actually changed it by one full year. This is entirely non-evident from the "explanation" on page 239.
Furthermore, in non-English editions of The Truth Shall Make You Free published a year or more after 1943, the explanations about changing 606 B.C. to 607 B.C. were replaced with explanations about changing 607 B.C. to 608 B.C., except that the end result remained that the Gentile times began in 607 B.C. This is akin to explaining to someone in English that 3+5=10 and in German that 4+5=10. Both explanations are not only wrong, but incompatible with one another.
Such is arithmetic as practiced by the Watchtower Society. As one JW once told me, after realizing what the Society had done, "it's nothing but a shell game."