The WT starts its Seventy Years with the moment that Jews entered Egypt following the murder of Governor Gedaliah. The WT guesses that the destruction and the murder of Gedaliah took place in the same year. However, the events listed in the Bible as taking place from the time of the city's destruction until the entry in Egypt require far more than two months. The period from the destruction to the entry into Egypt was likely 2, 3 or even 4 years long.
A discussion in Italian on the subject can be seen at:
http://www.infotdgeova.it/storia/quando70.php
The PDF file is available at:
http://www.infotdgeova.it/downloads/70anni.pdf
Doug