A date that the WTS - and everyone else - is incapable of proving is the timing when the first returnees gathered at the site of the ruined temple. The WTS arbitarily says this took place in 537 BCE while others, just as arbitarily, select other dates.
Without any Biblical justification, the WTS ends its "70 years" with that event. From there, it jumps back 70 years to the exodus into Egypt by the murderers of Gedaliah.
Interestingly, although the WTS requires the land to be utterly depopulated for the entire 70 years, the returnees were settled in their homes before they trekked to Jerusalem. Only then does the WTS end its "70 years".
Doug