As certain as any date could be. This is evident by various statements including the following:
1) “The date 1925 is even more distinctly indicated by the Scriptures because it is fixed by the law God gave to Israel.”
Watchtower, Sept 1, 1922, p.262
2) “Our thought is, that 1925 is definitely settled by the Scriptures, marking the end of the typical jubilees.”