The JWs have a problem because they have determined that Satan was cast out of Heaven in 1914, and at the same time, Jesus "Presence" was commenced over the earth. Their "evidence" is based on an interpretation of Prophecy sequencing that had to commence in 607 BCE in order to lead up to 1914. See their literature on Daniel's prophecy for details.
The Witness error places the conquest of Jerusalem under Nebuchadnezzar twenty years too early. The event, according to both the Bible and historical records, could not have happened in 607 BCE because Nebuchadnezzar had not yet begun to reign. He did not start his reign until two years later, in 605 BCE.
On the other hand, historical records from both inside and outside the Bible jibe nicely, putting the fall of Jerusalem at 19-20 years into Nebuchadnezzar's reign, at 586 BCE. There is credible support for 607. Raymond Franz details his "Crisis of Conscience" when the Watchtower Society was persuading him to overlook the "tweaking" of the dates. The Brooklyn based directors of the Witness movement wanted him to falsify the dating of history. He took the problem to the top and was eventually disfellowshipped, as his book reveals.
Why did the Society deem it so important? They had decided on certain events happening in 1914. Counting backwards in time, the Weeks of Daniel's prophecy led them errently back to the seventy years of the Babylonian Captivity and 607.
However, all the juggling is not necessary. The accurately dated events from Bible and history are as follows:
BCE 586 Jerusalem Falls
BCE 538 Babylon Falls to Cyrus (JWs in error count back c. 70 years from this date.)
BCE 538 Cyrus gives permission for captives to return.
BCE 516 Cyrus' permission is fully acted upon. This was prophesied in Jeremiah 25:10-12.
Even scholars who do not believe the Bible do not dispute the above historical dates. In many cases they use Bible clues to help with historical dates and are not disappointed. On the hand, they do not accept "tweaking" of ageed upon facts. Neither should Bible believers.