I believed in 1975 we were 6000 years into the 7th creative day and that the generation that saw the events of 1914 would not pass away before the end came.
When 1975 went I had no doubts regarding the 'truth' as I believed as the light gets brighter things become more clear and there had been things in the past like 1925.
I believed that it was right to join the anointed in the preaching work before the Great Tribulation strikes.
My belief came from study and personal experiences. In the early 90's it changed and I felt I could no longer be 100% sure in my mind of what I was preaching due to a Watchtower article that related to an experience in the ministry and so I stopped.