From JW org
“Six Thousand Years of Human Existence”
In 1969 the Watchtower magazine in Czech began publishing a series of articles based on the book Life Everlasting—In Freedom of the Sons of God. Chapter 1, under the subheading “Six Thousand Years of Human Existence Closing,”contained an explanation of the Jubilee and of Bible chronology. This material influenced some in a positive way; it also led to many questions and much speculation.
The office in Czechoslovakia sent a letter dated February 22, 1972, to all congregations. It set out a lengthy explanation of reasons why we should not make any definite assertions about the date when Armageddon will strike. It pointed out that no publication of the Society had said that Armageddon will come in a certain year. The letter concluded: “Jehovah’s Witnesses around the world are familiar with these facts, and no one should add any personal claims as to what will happen before or during the year 1975. There are no Scriptural grounds for any claims, and they could have a detrimental effect on the preaching work. Strive, therefore, that you ‘all speak in agreement and that there be no divisions among you but that you may be fitly united in the same mind and in the same line of thought.’ (1 Cor. 1:10) For concerning that day and hour nobody knows.”—Matt. 24:36.
Also read this website
http://defendingjehovahswitnesses.blogspot.com/2011/09/1975-and-jehovahs-witnesses-opposers.html