in the book Jehovah's Witnesses Proclaimers of God's kingdom (1993 WTB&TS) on page 12/13 it says,
So a witness relate facts from direct personal knowledge, or he proclaims views or truths of which he is convinced.*
*For example, some first-century Christians could bear witness to historical facts about Jesus-concerning his life, death, and resurrection- from firsthand knowledge. (Acts1:21,22;10:40,41) However, persons who later put faith in Jesus could bear witness by proclaiming to others the significance of his life, death, and resurrection-Acts22:15.
I believe the witnesses have proclaimed the establishment of God's Kingdom in 1914 therefore I don't see how they could change it although recently an elder told me if it was 20 years later it wouldn't matter. i think it would.