I've been hearing about 1975 for decades but i've never found a publication where this date is explicitly written.
When I Googled 'explicitly' it had this.
in a clear and detailed manner, leaving no room for confusion or doubt.
"the essay should state explicitly how the facts support the thesis"
My view is this, the way I understood it at the time, 1974. When I started regular pioneering. Creative days were 7,000 years long including the 7th. The 7th began in 4026 BCE.
"The first half or 'evening' period of God's seventh creative 'day' was now closing, 3,500 years from the creation of Adam and Eve. The morning of this creative 'day' was due to begin at 526 B.C.E." (God's Eternal Purpose Now Triumphing for Man's Good 1974 WTB&TS, page 131)
Page 51 of the same book there is a subheading "Evening of the Seventh Creative Day begins 4026 B.C.E."
Also, I understood that Armageddon would be over finished before the thousand year reign of Christ began.
Page 186 same book,
"By removing the Serpent and his demons from the neighborhood of the earth and hurling them into the abyss and sealing them up there, bound as with chains for the next thousand years. Revelation 20:1-3 pictures this, not as a part of the war at Har-magedon, but as a follow up of that war."
We also offered this publication in our ministry, as is mentioned in the feature of the Kingdom Ministry October 1974, Presenting The Good News.
So in 1974 studying the new publication and offering it in field service the conclusion I came to is that Armageddon would be over by the autumn of 1975
In the August 15, 1968, Watchtower in the article, Why are you looking forward to 1975? It shows the math and the year zero
Well, for one thing, if 4026 is added to 1968 (allowing for the lack of a zero year between C.E. and B.C.E ) one gets a total of 5,993 years, come this autumn from Adam's creation. That means that, in the fall of the year 1975, a little over seven years from now..., it will be 6,000 years since the creation of Adam,