The Watchtower book Revelation - Its Great Climax is at Hand! (1988) quotes the old Barbour booklet on page 105:
"'It was in B.C. 606, that God's kingdom ended, the diadem was removed, and all the earth given up to the Gentiles. 2520 years from B.C. 606, will end in A.D. 1914.' - The Three Worlds, published in 1877, page 83."
A footnote informs us:
"Providentially, those Bible Students had not realized that there is no zero year between "B.C." and "A.D." Later, when research made it necessary to adjust B.C. 606 to 607 BC, the zero year was also eliminated, so that the prediction held good at "A.D. 1914." - See "The Truth Shall Make You Free," published by the Watch Tower Society in 1943, page 239."How do you MOVE a year?
"Providentially" - by God's direction - they had made two errors cancelling each other out!
Apparently 2 wrongs do make a right!
Barbour was, like Russell, very ignorant about chronology and history. When the calendar we use (Julian, later improved to the Gregorian) was created, there was no zero year, so 1 BC was followed by 1 AD. 2520 years from 606BC would give us 1915, which Russell actually used instead of 1914 for a few years. But then World War I started in 1914. Russell had predicted something for 1914, something did indeed happen, so Russell thought he had been right. Russell then 'forgot' about the zero-year, and changed the chronology to 1914 again. (From Tower Watch Ministries)