"We need not here repeat the evidences that the 'seventh trumpet' began its sounding in A.D. 1840, and will continue until the end of the time of trouble." -The Watchtower Reprints, November 1890, page 1
Later, the Watchtower Society dropped this date and settled for a new one.
"The trumpet blast of the seventh angel was reflected in highlights of the Bible Students' convention in Detroit, Michigan, July 30-August 6, 1928." -Revelation, It's Grand Climax At Hand, page 172
See this page with a large collection of quotes about the Watchtower's understanding of the seven trumpet blasts.