1) Ask a JW to please read Zechariah 1:12:
So the angel of Jehovah answered and said: "O Jehovah of armies, how long will you yourself not show mercy to Jerusalem and to the cities of Judah, whom you have denounced these seventy years." (NWT)2) Ask what the date of destruction was. They always reply 607 BCE.
3) Then request they look in their bible, and give the date when their bible (New World Translation) says (correctly) that the book of Zechariah was written. They will reply 520-518 BCE.
4) Request the witness to do some simple mathematics asking, what is 520-518 BCE added to 70. The obvious answer is 590-588 BCE maximum. (As we know due to regnal years etc. the closest date and correct date is 587/588)
5) Ask them, if the teaching concerning 607 BCE by the faithful and discreet slave is true, how can the bible they carry deny it!!!
If nothing else, it will cause a severe doubt of the date, which holds all things together for the Jehovah's Witnesses. I can also tell you they do not have one book which deals with this problem, and have even gone as far, in most of their bibles, to omit any cross reference to this scripture. This scripture is the one that blew it wide open for me and started me searching for truth and the path out.