Russel first concluded that Jesus had already returned invisibly to earth in 1799, and that Armageddon would come in 1874. It didn't happen, so Russel recalculated and pointed to 1914 as the date of Armageddon. This was based on the calculation of the "Gentile Times" of Daniel. Russel believed that Jerusalem was destroyed in 606BCE, but then changed it to 607BCE, a date that no scholar outside the Jehovah's Witnesses accepts. Archaeological evidence proves that Jerusalem was destroyed by Nebuchadnezzar II in 586BCE, twenty-one years later.
Even though the 607 date is blatantly false, Jehovah's Witnesses continue to believe it, because if the organization were to accept the 586 date, that would put Christ's invisible return at 1935, which is not as "sexy" as 1914. Nothing of worldwide note happened in 1935, unlike the outbreak of World War I in 1914.
The Witnesses have had to change many of their timetables due to embarrassment of the "end" not coming. They have also pointed to 1925, 1944, and 1975 as the year of Armageddon, all to no avail. Their teaching that the "generation" who saw 1914 would not pass away had to be adjusted in 1995 to become much more ambiguous.
