The secular sources that provide the WTS with the 539 BCE date employ a system that commences with Absolute Dates, such as 568 BCE for the 37th year of Nebuchadnezzar, and then employing the secular accepted neo-Babylonian chronology to arrive at the intervening dates.
If the WTS tries to locate a precise and exact "70-year period", then it does not understand the mind of the ancient Hebrews or their historiography. Trying to do so imposes 21st century thinking into the minds and perceptions of an agrarian people living in a different culture some 2000 to 2500 years ago.
Today, we might think that the record of history is a tabulation on facts, records of diaries, and so on, but in those days the recording of history was something new and untried. These "historians" were religious scribes whose purpose was religious. For example, to think that the Gospel accounts are literal biographies is to completely misunderstand them. Each is a religious construct by a particular group written for its own group to suit its own religious ends. The earliest - Mark - was written 40 years after Jesus walked, talked, and was executed.
Doug