As I think you know, the WT's interpretation of the "70 years" referred to by Jeremiah makes no sense. For several reasons.
One is that Jeremiah 25:11,12 tells us, in part, that "these nations (plural - not just Judah) will have to serve the king of Babylon 70 years," and "when the 70 years are completed I will punish the king of Babylon and his nation."
History tells us that in 609/608 BCE, during the reign of Nebuchadnezzar's father Nabopolassar, Babylon replaced Assyria as the dominant political and military power of the ancient Near East.( Shortly thereafter, in 607/606 BCE, crown prince Nebuchadnezzar began a long series of attacks against Judah as well as Babylon's other neighboring nations. ) Seventy years, or parts thereof, after Babylon's rise to power, In 539 BC, Babylon was apparently "punished" by God for its actions when it was overthrown by Persia's Cyrus the Great.
The Watchtower Society tells us that Jeremiah's "70 years" prophecies began to be fulfilled in 607 BCE, when they say Judah was completely desolated by Babylon. And they tell us the 70 year period of time Jeremiah prophesied of ended when the Jewish people returned to their homeland 70 years later, in the year 537 BCE.
So, in defense of the Watchtower Society's understanding of Jeremiah's "70 years" prophecy, JWs must explain Jeremiah 25:12. There we read, "When the seventy years have been fulfilled I will call to account against the king of Babylon and against that nation." (NWT) Or, as the New American Standard Bible says, "When the seventy years are completed I will punish the king of Babylon and that nation." So, the Bible says that Babylon would not be "called to account" or "punished" until the 70 years were "fulfilled" or "completed." But all reasonable Bible students, JWs and non-JWs, agree that Babylon was "punished" in 539 BCE.
So, all non-JW Bible readers who are familiar with history understand that the "70 years" Jeremiah spoke of in Jer. 25:11,12 ended in 539 BCE. But the Watchtower Society says they did not end until 537 BCE. So, according to the Watchtower's interpretation of scripture, God punished Babylon two years before the 70 years ended, even though He plainly told Jeremiah He would not do so until the seventy years were "completed."
In Daniel chapter 9 we find that Daniel had been studying Jeremiah's "70 years" prophecies pertaining to what would happen to both Babylon and the Jewish people after the 70 years God had allowed Babylon to dominate its neighboring nations came to an end. Daniel had no doubt been reading Jer. 25:6,7 which quoted God as having said, "I will hand all your countries over to my servant Nebuchadnezzar king of Babylon ... All nations will serve him and his son and his grandson until the time for his land comes; then many nations and great kings will subjugate him." And verses 11,12 of the same chapter where God said, "These nations (plural - not just Judah) will have to serve the king of Babylon 70 years. ... And when the 70 years are completed I will punish the king of Babylon and his nation." And Jer. 29:10,11 where God said, "When seventy years are completed for Babylon, I will come to you and fulfill my gracious promise to bring you back to this place. For I know the plans I have for you," declares the Lord, "plans to prosper you and not to harm you, plans to give you hope and a future."
To what exact period of time Jeremiah's "70 years" were meant to apply is a bit uncertain. What is certain, however, is that the Watchtower's application cannot be correct because it clearly conflicts with the scriptures themselves. As mentioned earlier, Historians tells us that in 609/608 BCE Assyria ceased to exist and her territory was taken over by the Babylonians. Historians also tells us that in 607/606 BCE crown prince Nebuchadnezzar led his first military campaign against Babylon's neighboring nations. And they tell us that in 539 BCE, Babylon was apparently punished by God for its actions, as Jeremiah had prophesied, when it was overthrown by Persia's Cyrus the Great.
In resolving this matter it might be helpful to remember that the Jews at the time of Jeremiah (as they still do today) used a lunar calendar containing 354 days. So, if Jeremiah's "70 year" prophecies pertaining to Babylonian domination of Judah and her neighboring nations referred to 70 solar years they may be counted from 609 to 539 BCE. However, if they referred to 70 lunar years (which would equal only 68 solar years) they may be counted from 607 BCE (the year Nebuchadnezzar himself first began to subjugate Babylon's neighboring nations) to 539 BC. The 70 lunar years application may actually fit the facts of history and scripture better than the 70 solar years application. For God had said, "I will hand all your countries over to my servant Nebuchadnezzar king of Babylon ... All nations will serve him and his son and his grandson until the time for his land comes." (Jer. 25:6) So the actual wording of Jeremiah's "70 years" prophecy concerning Babylon's domination of its neighboring nations does seem to have dictated a starting point when Nebuchadnezzar himself would begin to exercise such control.
In any case, the Watchtower's interpretation does not fit either the scriptures or the facts of history.