Some references of past pronouncements on the timing. Probably why they've given up and just put 'about 1914' in the RNWT appendix:
w00 1/1 p. 8 par. 10 Serving With the Watchman
True to Bible prophecy, this world’s “conclusion” began in 1914 and was marked by the outbreak of the first world war. (Matthew 24:3, 7) There is strong evidence that Satan and his demonic hordes were cast out of heaven following that year.[1915, then]
w83 5/15 pp. 22-23 pars. 5-6 The Glorious Work That Follows the Ride of the Horsemen
Since the fiery-colored horse and its war-minded rider followed the rider on the white horse, does this mean that the first horseman started World War I after his coronation in 1914 and thus started using his “bow”? Not at all! Revelation chapter 12 pictures what would follow the birth of the Kingdom of the first horseman in 1914 [what would follow after October 1914]: an invisible, superhuman war. “And war broke out in heaven: Michael and his angels battled with the dragon, and the dragon and its angels battled but it did not prevail, neither was a place found for them any longer in heaven. So down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; he was hurled down to the earth, and his angels were hurled down with him. And I heard a loud voice in heaven say: ‘Now have come to pass the salvation and the power and the kingdom of our God and the authority of his Christ, because the accuser of our brothers has been hurled down, who accuses them day and night before our God! . . . On this account be glad, you heavens and you who reside in them! Woe for the earth and for the sea, because the Devil has come down to you, having great anger, knowing he has a short period of time.’”—Verses 7-12.
6 That account places the blame for the first world war [which preceded October and Satan's ousting and wrath] that claimed more than 8 million human lives from July 28, 1914, squarely upon the Devil and his demons.
w70 12/15 p. 757 par. 28 End of Worldwide Witnessing Gets Nearer
After his enthronement in heaven [October 1914], World War I on earth was overshadowed by the fact that “war broke out in heaven” and Satan was cast down to the earth [sometime after October, therefore].—Rev. 12:1-13.
Finished Mystery (1969), p. 314 Birth of God's Messianic Kingdom
The ouster of the 'great dragon' and his demons from heaven was apparently fully accomplished by the end of World War I [by 1918 - lots of wiggle room there].
w53 2/15 p. 119 par. 23 When Will God’s Kingdom Come?
God knew that Satan the wicked one would attempt to turn the attention of the people away from God’s kingdom when it was established. So it was when Satan was ousted from the heavens down to the earth. [that'll be 1914] He was wrathful. He plunged the nations into World War I. [in August] He wanted to turn the attention of the people away from the significant event that then was taking place [in October] and he proceeded to do so in the most deceptive ways. But he must hurry! Why? Listen to the inspired account at Revelation 12:9, 12 (NW): “So down the great dragon was hurled, the original serpent, the one called Devil and Satan, who is misleading the entire inhabited earth; . . . Woe for the earth and for the sea, because the Devil has come down to you, having great anger, knowing he has a short period of time.”