The WTS says that the 70 years began when the last Jews entered Egypt (7th month, Feast of Gedaliah), and that this final exodus took place in 607 BCE.
They then say that Jerusalem was destroyed 2 months earlier (5th month) and that this event marked the end of the Davidic kingdom, which commenced the 2520 years. On that basis, if my reading is correct, their 2520 years ended on the 5th month of 1914, not in October.
Further confusion reigns, since according to Parker and Dubberstein, in 607 BCE the Babylonians inserted an intercalary second Elul before Tishri. From memory, Thiele says that Jeremiah used the Babylonian calendar.
I know that their chronology and eisgeses are totally fanciful; I am simply addressing the logic in the thinking that they appear to present.
I have elsewhere shown that it is more likely that the time fom the destruction of Jerusalem to the exodus into Egypt is more likely to have been at least 2 years, perhaps as long as 4, so I am not canvassing that issue here either.
Doug