The WT Proclaimers book said the following which brought out new dates I knew nothing about.
https://wol.jw.org/en/wol/d/r1/lp-e/1101993039#h=67:0-67:669
Using Bible chronology that had first been laid out by Christopher Bowen of England, they thought that 6,000 years of human history had ended in 1873, that thereafter they were in the seventh thousand-year period of human history, and that they had surely approached the dawn of the foretold Millennium. The series of books known as Millennial Dawn (and later called Studies in the Scriptures), which were penned by C. T. Russell, drew attention to the implications of this according to what the Bible Students understood from the Scriptures.
Something else that was seen as a possible time indicator involved the arrangement that God instituted in ancient Israel for a Jubilee, a year of release, every 50th year. This came after a series of seven 7-year periods, each of which ended with a sabbath year. During the Jubilee year, Hebrew slaves were freed and hereditary land possessions that had been sold were restored. (Lev. 25:8-10) Calculations based on this cycle of years led to the conclusion that perhaps a greater Jubilee for all the earth had begun in the autumn of 1874, that evidently the Lord had returned in that year and was invisibly present, and that “the times of restitution of all things” had arrived.—Acts 3:19-21, KJ.
Based on the premise that events of the first century might find parallels in related events later, they also concluded that if Jesus’ baptism and anointing in the autumn of 29 C.E. paralleled the beginning of an invisible presence in 1874, then his riding into Jerusalem as King in the spring of 33 C.E. would point to the spring of 1878 as the time when he would assume his power as heavenly King.* They also thought they would be given their heavenly reward at that time. When that did not occur, they concluded that since Jesus’ anointed followers were to share with him in the Kingdom, the resurrection to spirit life of those already sleeping in death began then. It was also reasoned that the end of God’s special favor to natural Israel down to 36 C.E. might point to 1881 as the time when the special opportunity to become part of spiritual Israel would close.*