Russell thought that Adam was created 4127 BCE, so 7000 years would end in 2874 CE. Since he should rule for 1000 years, this reign must have started in 1874.
Since nothing happened in 1874, he concluded that there would be period of 40 years (like 30 to 70 CE or like the 40 years Moses spent in the wilderness), and Armageddon would come 40 years after 1874 (= 1914).
Since nothing happened in 1914, they eventually "noticed" that Adam was not created in 4127 but in 4026, pointing to 1975 instead of 1914. So they changed 1914 from "beginning of 1000 year reign" to "beginning of invisible rein over his congregation".
SInce nothing happened in 1975, they eventually "noticed" that the 6th day did not end with Adam's creation but with Eve's creation. So they changed it from "1975" to "soon", which is valid until now.
Since there is overwhelming evidence that the earth is more than 49,000 years old, they silently dropped the idea that one creative day would have been 7,000 years.