v.21 reads:
The land enjoyed its sabbath rests; all the time of its desolation it rested, until the seventy years were completed in fulfillment of the word of the LORD spoken by Jeremiah.
It is important to note that the 70 years are to be completed in fulfillment of the word of the Lord spoken by Jeremiah. It would not be correct to try to come to a conclusion about the meaning of this verse, without considering the words of Jeremiah. Where can we find this words of Jeremiah? They are in Jeremiah 25:11b, 12 (NWT): "11b and these nations (the list of "these nations" you find in v.17-26, demonstrating it was not just about Juda & Jerusalem) will have to serve the king of Babylon seventy years.”’ 12 “‘And it must occur that when seventy years have been fulfilled I shall call to account against the king of Babylon and against that nation,’ is the utterance of Jehovah, ‘their error, even against the land of the Chal·de′ans, and I will make it desolate wastes to time indefinite."
After a period of 70 years during which Juda and all the nations around Babylon had to serve the Babylonians, God would bring Babylon to its end. Historians and JW all agree that this happened in October 539 BC, an event well documented in Daniel 5. So the 70 years in 2 Chron 36:21, are 70 years for Babylon, and not for Jerusalem. The verse specifies that the end of the sabbath rest is determined by the end of the supremacy of Babylon. The end of the Sabbath rest of Jerusalem coincided with the end of Babylon, but the beginning of the Sabbath rest did not have to coincide with the beginning of the 70 years for Babylon. the sabbath rest started with another moment, the destruction of Jerusalem.
Historians found that the beginning of the period of servitude of the nations to Babylon, or in other words, Babylon's supremacy in the middle east, started in the 17th year of Nabopalassar, the father of Nebuchadnezar. The Supreme power of the middle east until then was Assyria. When Nabopalassar captured Haran, and defeated Assyrias last king Usharballit, Babylon became the supreme power in the middle east. This information you also find in "Insight on the Scriptures", under the heading "Assyria" (suprisingly not Babylon). The siege and capture of Haran is dated by historians to 609 BC. if we add to this 70 years, we end up exactly in 539 BC. Fullfilling exactly all the related prophecies.
Counting the king's years from the 17th year of Nabopalassar till the fall of babylon, will also give you exactly 70 years, confirming Jeremiah's words.
I hope this helps you further
Hoffnung