“These nations will serve the king of Babylon seventy years. But when the seventy years are fulfilled, I will punish the king of Babylon and his nation, the land of the Babylonians, for their guilt,” declares the Lord, “and will make it desolate forever. I will bring on that land all the things I have spoken against it, all that are written in this book and prophesied by Jeremiah against all the nations. They themselves will be enslaved by many nations and great kings; I will repay them according to their deeds and the work of their hands.” (Jeremiah 25:11-14)
A few years after the Assyrian Empire collapsed, an extraordinary prediction was made by the prophet Jeremiah. He said that the incoming empire of Babylon would last seventy years then it too would end. His word came true to the very year! Babylon fell to the Medes and Persians in October 539 BC, seventy years after the demise of Assyria in 608 BC. (Nineveh was lost in 612 BC but Assyria's last stand was the city of Harran four years later.)

That is not all. A dual fulfilment aspect to the prophecy had Babylon's duration as described, but also had Jerusalem lying desolate for a concurrent 70 years. Now, the two predictions were offset from each other because Babylon's start was 608 BC, whereas the siege of Jerusalem started in December 589 BC. So, there are actually two counts of seventy, the second count lasting until 520 BC when Darius endorsed a decree to commence the rebuilding of the temple.
His decree was made Oct/Nov 520 BC, and on December 18th, Haggai marked it as the end of Jerusalem's punishment.
"The vine, the fig tree, the pomegranate, and the olive tree have yielded nothing, but from this day on I will bless you.” (Haggai 2:19)
When we count from the beginning of the siege to Darius's decree, once again there are seventy years. Unfortunately, some commentators have added up sixty-nine years, describing the fulfilment as, “approximate.” It is unfortunate because “near enough” is not the way God works. Others have construed 69 solar years to mean 70 ‘prophetic’ years of 360 days, but alternative calendars are not needed either.

Counting time in the Bible is inclusive. If the 1st year only consists of a few months before the next New Year, it is still deemed to be the 1st year. Likewise, the last year is a ‘year.’ In other words, part-years are always treated as full years when reckoned on the old Hebrew calendar, and this is demonstrated on numerous occasions throughout the Bible. Such is the case here and it is indeed seventy years.

One of the reasons ‘Jeremiahs 70 years’ is not often quoted as an accurate example of predictive prophecy is because its details are spread over 4 or 5 different references and historical events. In the following diagram I have bought them together so that its extraordinary accuracy can be appreciated.