Apologies for the length of this introduction but I felt it was important to begin with a weight of evidence. There have been a great many changes in the Jehovah's Witness understanding and prophetic predictions regarding the commencement and conclusion of the 'time of trouble', the beginning of Christ's 'second presence' and His coming into the Kingdom with power, the length and composition of 'this generation', and even which people group all these things apply to.
I have bolded parts of each of the references below. One of the most dramatic changes is that 1799 marks the beginning of the 'time of the end' and then 1914 is given as the end and not the beginning of the time, and then 1914 is given as the beginning of the time. At this point 1914 is clung to and the definition of 'this generation' begins to change.
"...1799 definitely marks the beginning of 'the time of the end'.... 'The time of the end' embraces a period from A.D. 1799, as above indicated, to the time of the complete overthrow of Satan's empire....we have been in 'the time of the end' since 1799" (The Harp of God, 1928 ed., pp. 235-36, 239).
"The time of the Lord's second presence dates from 1874....From 1874 forward is the latter part of the period of 'the time of the end'. From 1874 is the time of the Lord's second presence... It was in the year 1874, the date of our Lord's second presence..." (The Harp of God, 1928 ed., pp. 236, 239-40).
"CAN IT BE DELAYED UNTIL 1914?...our readers are writing to know if there may not be a mistake in the 1914 date. They say that they do not see how present conditions can last so long under the strain. We see no reason for changing the figures - nor could we change them if we would. They are, we believe, God's dates not ours. But bear in mind that the end of 1914 is not the date for the beginning, but for the end of the time of trouble" (Watch Tower, 15 July 1894, p. 226).
"The Scriptures indicate that a great time of trouble similar to that which came upon the Jewish nation will now come upon all Christendom. The experience of Israel in the year 70 [destruction of Jerusalem] will be paralleled in the experiences of the year 1915" (Watch Tower, 15 June 1913, p. 181).
"...in the year 1918, when God destroys the churches wholesale and the church members by millions, it shall be that any that escape shall come to the works of Pastor Russell to learn the meaning of the downfall of 'Christianity'" (The Finished Mystery, 1917, p. 485).
"The date 1925 is even more distinctly indicated by the Scriptures because it is fixed by the law God gave to Israel. Viewing the present situation in Europe, one wonders how it will be possible to hold back the explosion much longer; and that even before 1925 the great crisis will be reached and probably passed" (Watch Tower, 1 September 1922, p. 262).
"As we have heretofore stated, the great jubilee cycle is due to begin in 1925. At that time the earthly phase of the kingdom shall be recognized... Therefore we may confidently expect that 1925 will mark the return of Abraham, Isaac, Jacob and the faithful prophets of old, particularly those named by the Apostle in Hebrews chapter eleven, to the condition of human perfection" (Millions Now Living Will Never Die! pp. 89-90).
"The foretold events having begun A.D. 1914, the generation of mankind that is still alive from that year is the generation meant by Jesus Christ. Till now we have seen fulfilled the world-shaking features of the 'sign of [Christ's invisible] presence and of the consummation of the system of things.'...The occurrence of these things since 1914 is within the knowledge of millions of this generation.... We, then, are the generation that will not pass away till there is fulfilled that 'great tribulation such as has not occurred since the world's beginning until now, no, nor will occur again'" (Watchtower, 15 April 1961, p. 236; brackets in original, italics added).
"Jesus said 'this generation will by no means pass away until all these things occur.' Which generation is this, and how long is it?... [W]hen it comes to the application in our time, the 'generation' logically would not apply to babies born during World War I. It applies to Christ's followers and others who were able to observe that war and the other things that have occurred in fulfillment of Jesus' composite 'sign.' Some of such persons 'will by no means pass away until' all of what Christ prophesied occurs, including the end of the present wicked system" (Watchtower, 1 October 1978, p. 31).
"If Jesus used 'generation' in that sense and we apply it to 1914, then the babies of that generation are now 70 years old or older. And others alive in 1914 are in their 80's or 90's, a few even having reached a hundred. There are still many millions of that generation alive. Some of them 'will by no means pass away until all things occur.' - Luke 21:32" (Watchtower, 15 May 1984, p.5).
"To calculate where man is in the stream of time relative to God's seventh day of 7,000 years, we need to determine how long a time has elapsed from the year of Adam and Eve's creation in 4026 B.C.E. From the autumn of that year to the autumn of 1 B.C.E., there would be 4,025 years. From the autumn of 1 B.C.E. to the autumn of 1 C.E. is one year (there was no zero year). From the autumn of 1 C.E. to the autumn of 1967 is a total of 1,966 years. Adding 4,025 and 1 and 1,966, we get 5,992 years from the autumn of 4026 B.C.E. to the autumn of 1967. Thus, eight years remain to account for a full 6,000 years of the seventh day. Eight years from the autumn of 1967 would bring us to the autumn of 1975, fully 6,000 years into God's seventh day, his rest day.(Watchtower, 1 May 1968, pp. 271-72).
An April 8, 1988 issue of Awake magazine, in an article entitled 'How Long Can a Generation Last?' intimated that 1989 would be the end of the 'last generation'
The 1994 Watchtower 2 volume research dictionary, "Insight on the Scriptures" calculated a possible end to the 'last generation' as 1994. (Vol. 1, pp. 917-18).
Finally in Watchtower (1 November 1995, p. 19) the definition of 'this generation' was changed from those born in 1914 to "all the peoples of earth who see the sign of Christ's 1914 presence but fail to repent".
The historical pattern of the Watchtower Bible and Tract Society appears to be one of thresholds proclaimed, reached, exceeded, and changed. Rinse and repeat. What is the Jehovah's Witness explanation of this constantly evolving eschatology surrounding the year 1914?