The WTS cant arrive at 539 (the agreed pivotal date) without relying on external "secular" sources, there is no internal absolute dating in the bible that allows anyone to date this thing, so they have to go external even for that.
Off the top of my head, there's zechariah 7: 1-5 that mentions 70 years (since the temples destruction), but it is in the reign of Darius (518) so the bible doesnt allow for it to be 607 just based on that one verse. (518 +70 = 587ish)
But as said above, cult mind control etc wont allow some jw cult drones to accept any amount of evidence.