This is a very important observation, that The Watchtower has not tried to explain in print why 607 BCE is the year for the destruction of Jerusalem for the last 17 years!!!
All that The Watchtower has done over most of the last two decades is to repeat over and over the year "607BCE" in its publications as if it is an unquestioned fact!
The LAST time an explaination for the year was attempted by The Watchtower was in 1988 when the "Insight" volumes were published, but since then, no explaination for the validity of the year has appeared in print.
This must mean something, but what? Perhaps that The Watchtower doesn't have anything new to say on the topic, or perhaps that they don't want people delving too deep in researching the dates 608/587 BCE. After all, COJ's book, The Gentile Times Reconsidered, is easily found by anyone starting to research the matter, and The Watchtower certainly does not want anyone finding out about that book!
--VM44