Jehovah's Witnesses teach that the last days started in 1914 because that was the year that the world changed from the start of WW1. The drastic changes from 1914 onward are the sign of the Lord's presence. Those who saw the sign starting in 1914 were the first of a generation that would not pass away before the last days ended. How do JWs define the term "generation" as used here?
The following is found on the JW.org website. It is from 2014. Prior to this it was taught that some of the group of Christians who were old enough in 1914 to comprehend the significance of the events that proved the messiah's second presence (parousia) had begun in that year would still be alive when the great tribulation starts.
The material below is quoted from the 2014 Watchtower article, "'Let Your Kingdom Come'—But When?"
In his detailed prophecy about the conclusion of this system of things, Jesus said: “This generation will by no means pass away until all these things happen.” (Read Matthew 24:33-35.) We understand that in mentioning “this generation,” Jesus was referring to two groups of anointed Christians. The first group was on hand in 1914, and they readily discerned the sign of Christ’s presence in that year. Those who made up this group were not merely alive in 1914, but they were spirit-anointed as sons of God in or before that year.—Rom. 8:14-17.
The second group included in “this generation” are anointed contemporaries of the first group. They were not simply alive during the lifetime of those in the first group, but they were anointed with holy spirit during the time that those of the first group were still on earth. Thus, not every anointed person today is included in “this generation” of whom Jesus spoke. Today, those in this second group are themselves advancing in years. Yet, Jesus’ words at Matthew 24:34 give us confidence that at least some of “this generation will by no means pass away” before seeing the start of the great tribulation. This should add to our conviction that little time remains before the King of God’s Kingdom acts to destroy the wicked and usher in a righteous new world.—2 Pet. 3:13.
This is what Jehovah’s Witnesses believed in 1984 regarding the meaning of the 1914 generation:
“This generation [of 1914] will by no means pass away until all things occur.” (Luke 21:32) And Jehovah, who is the source of inspired and unfailing prophecy, will bring about the fulfillment of his Son’s words in a relatively short time.—Isaiah 46:9, 10; 55:10, 11.
Just as Jesus’ prophecies regarding Jerusalem were fulfilled within the life span of the generation of the year 33 C.E., so his prophecies regarding “the time of the end” will be fulfilled within the life span of the generation of 1914. (Daniel 12:4) This means that marvelous prospects lie before not only that generation but all those living today. Why? Because Jesus also said regarding the significant events affecting that generation: “When you see these things occurring, know that the kingdom of God is near.”—Luke 21:28, 31.
The nearness of God’s Kingdom today spells the end of the present divisive political, religious and commercial systems. It means the ushering in of a righteous new government for all obedient mankind. You can choose everlasting life under this arrangement of “new heavens and a new earth.” (2 Peter 3:13; John 17:3) Yes, you may live to see this promised New Order, along with survivors of the generation of 1914—the generation that will not pass away.
Source: Watchtower 15 May 1984, pp 4-7: 1914—The Generation That Will Not Pass Away https://wol.jw.org/en/wol/d/r1/lp-e/1984361?q=generation+1914&p=par
This is what Jehovah’s Witnesses believed in 2010 regarding the meaning of the 1914 generation:
13... Consider our understanding of those who make up “this generation” mentioned by Jesus. (Read Matthew 24:32-34.) To what generation did Jesus refer? The article “Christ’s Presence—What Does It Mean to You?” explained that Jesus was referring, not to the wicked, but to his disciples, who were soon to be anointed with holy spirit.* Jesus’ anointed followers, both in the first century and in our day, would be the ones who would not only see the sign but also discern its meaning—that Jesus “is near at the doors.”
14 What does this explanation mean to us? Although we cannot measure the exact length of “this generation,” we do well to keep in mind several things about the word “generation”: It usually refers to people of varying ages whose lives overlap during a particular time period; it is not excessively long; and it has an end. (Ex. 1:6) How, then, are we to understand Jesus’ words about “this generation”? He evidently meant that the lives of the anointed who were on hand when the sign began to become evident in 1914 would overlap with the lives of other anointed ones who would see the start of the great tribulation. That generation had a beginning, and it surely will have an end. The fulfillment of the various features of the sign clearly indicates that the tribulation must be near. By maintaining your sense of urgency and keeping on the watch, you show that you are keeping up with advancing light and following the leadings of holy spirit.—Mark 13:37.
Source: Watchtower 15 April 2010 pp7-11: Holy Spirit’s Role in the Outworking of Jehovah’s Purpose https://wol.jw.org/en/wol/d/r1/lp-e/2010282#h=17:220-18:787
In the 15 July 2013 Watchtower, the following admission was made:
"In the past, we thought that the great tribulation began in 1914 when World War 1 started. We thought that Jehovah 'cut short' those days in 1918 when the war ended so that the remaining anointed ones on earth could preach the good news to all nations. Then we realised that a part of Jesus' prophecy about the last days has two fulfilments. So we needed to change the way we understood some parts of the prophecy."
Kris has already extracted two paragraphs from the 2014 Watchtower article regarding the explanation for now having two groups of anointed Witnesses:
15 We understand that in mentioning “this generation,” Jesus was referring to two groups of anointed Christians. The first group was on hand in 1914, and they readily discerned the sign of Christ’s presence in that year. Those who made up this group were not merely alive in 1914, but they were spirit-anointed as sons of God in or before that year.
16 The second group included in “this generation” are anointed contemporaries of the first group. They were not simply alive during the lifetime of those in the first group, but they were anointed with holy spirit during the time that those of the first group were still on earth. Thus, not every anointed person today is included in “this generation” of whom Jesus spoke. Today, those in this second group are themselves advancing in years.
Source: 2014 Watchtower article, "'Let Your Kingdom Come'—But When?" https://wol.jw.org/en/wol/d/r1/lp-e/2014047#h=24
According to what Governing Body member David H. Splane said in this 2018 video clip https://www.jw.org/en/library/videos/#en/mediaitems/StudioTalks/pub-jwban_201509_1_VIDEO a generation is a group of contemporaries who have lived at the same time. For example, Joseph and all his brothers (Exodus 1:6) formed a generation. The generation Jesus spoke about (in Matthew 24:32-34) who would “see all these things” were the generation alive in 1914 who had the spiritual discernment to know that something invisible was occurring. Only the anointed remnant of Jehovah’s Witnesses had that spiritual discernment. Brother F.W. Franz, one of the anointed (and born in 1893) was the last of the FIRST group to discern what was going on in 1914. He died in 1992. Therefore, Mr Splane concludes, any of the remnant who were anointed before 1992 would be contemporaries of the first generation of anointed ones. The SECOND group of anointed ones are getting older, so we must be getting very close to the great tribulation which will culminate in “the end of this system of things.”
It is noteworthy that the original view that “the end of this system of things” would be witnessed by the generation alive in 1914 (those who understood what was going on in heaven) was “amended” after it became apparent that almost all of them had died. That is when the new teaching about the meaning of “generation” was introduced, that it was now an “overlapping” generation. Apparently, any Witness who was anointed before Brother F.W. Franz died in 1992 has become part of that overlapping generation. The official view is that the 1914 generation is still going strong, because contemporaries of F.W. Franz are still alive. The length of the generation has been changed from meaning those alive in 1914 who understood what was going on (spiritually speaking) to meaning the generation that has overlapped the 1914 generation.
No upper time limit for this “overlapping generation” has been given although from the quotes given above, that timeline cannot be stretched indefinitely: “That generation had a beginning, and it will surely have an end.” “...at least some of “this generation will by no means pass away” before seeing the start of the great tribulation.”
"The SECOND group of anointed ones are getting older, so we must be getting very close to the great tribulation which will culminate in “the end of this system of things.”
This is the current Jehovah's Witnesses interpretation of Jesus' words in Matthew 24:1–34, about the generation that would not pass away before great tribulation.