First they claimed 1914.
Watchtower 1997, July 1st, pg.7
All the evidence shows that since 1914 the great opposer of God, Satan, has been cast down to the vicinity of the earth, and the Kingdom is now operating as a reality!
Then they said: "Following That Year"
Watchtower 2000, January 1st, pg.8, par.10
"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."
Then they said: "About 1914"
In the New World Translation 2013 Edition, B1, pg. 1755, they said that Satan was cast out of heaven "About" 1914" Which suggests "not exactly" 1914, but somewhere around that year. Which could mean 1915 or some other date that they dream up.
Petra!