AND THE TRUE ANSWER IS:
Thanks A.J. for including this reference. There are THREE 70-year periods, all different.
Additionally, there is yet another 70 year period referred to in Zechariah 7:1-7. This passage says that the Jews had been fasting and wailing for 70 years since Jerusalem's destruction. This statement occurs in the 4th year of Darius, according to the passage. SO, if this corresponds with 517 BC (which is undisputed by the WTS), guess where 70 years prior to 517 BC puts us? ahem...
First to correct above, the 70 years that ends in the "4th year of Darius" is not 70 years from the destruction of Jerusalem but 70 years after the mourning in the fifth and seventh months, which is two years different than 70 years from the "denunciation of Jerusalem" at its destruction ending 2 years earlier in the 2nd year of Darius. By comparing these two 70-year periods, one ending in the 2nd and one ending in the 4th of Darius, we know they are two different 70-year intervals being spoken of. The 70 years ending after the "fasting" in the seventh month is two years different than 70 years from the fall of Jerusalem becaue Gedaliah was killed in the 20th year of Nebuchadnzzar, not the same year in the 7th month.
Gedaliah: The Bible does not say when Gedaliah was appointed. But he sent word out, after having assurances from the Babylonians that they intended to allow people in the land to harvest and told them to come in and harvest "summer fruits" including some "wine". These harvests started as early as the 4th month. Since it is reasonable to presume that some time is allowed to get the word out and relocate to Jersusalem, this was just before the summer crops when they were just getting ripe. Further the Bible suggests they came into the land in a gradual manner...
Jer. 40: 11 And all the Jews that were in Mo´ab and among the sons of Am´mon and in E´dom and those who were in all the [other] lands, they also heard that the king of Babylon had given a remnant to Judah and that he had commissioned over them Ged·a·li´ah the son of A·hi´kam the son of Sha´phan. 12 And all the Jews began to return from all the places to which they had been dispersed, and they kept coming into the land of Judah to Ged·a·li´ah at Miz´pah. And they went gathering wine and summer fruits in very great quantity.
All the above, of course, fit much more nicely into the time frame of occurring between the time Jerusalem was destroyed in the 5th month of the 19th year of Nebuchadnezzar and the spring of the following year, giving Gedaliah to be in place for a while and get reassurances from the Babylonians, and time for the people to gradually come into Jerusalem to start harvesting the wine crops which began in the 4th month. Thus he was killed in the 7th month of the following year after Jerusalem was destroyed. Word got back to Babylon about his being killed and they began to mourn him beginning the following year in the seventh month. That is why there is a a two-year difference between the 70 years from the denunciation (destruction) and the mourning in the seventh month. Therefore, threse two 70-year periods are different. How do they compare to the 70 years that end the exile?
They don't! That's because as noted already, the Jews were still waiting for God to show mercy to their cities and to Jerusalem and had promised he would "return" there and rebuild the temple. So we know they had not returned and the temple was not rebuilt yet. Showing "mercy to the cities" of course meant God allowing the people to return to them and rebuild them and bring them back to life again. That is what they are talking about. Since in the 4th of Darius the temple was already nearly rebuilt and the Jews had already returned from Babylon by now and even completed the walls of the city (completed in the 2nd year of Darius. Ezra 4:11), Zechariah is in no way a reference to Darius I but to Darius the Mede!
Thus the problem with understanding Zechariah again goes back to the revisionism of the Persians of the Babylonian Period in which they shortened it. Fortunately, we have Josephus' reference as to when the 70 years of Jeremiah was to be fulfilled found at Antiquities 11:1.1 which confirms the 70 years began with the last deportation in year 23 of Nebuchadnezzar (Jer. 52:30) and ended when the Jews were released from Babylon. This reference answers our puzzle!
If the 70 years of prophesied "servitude" did not begin until the last deportation in the 23rd year of Nebuchadnezzar, some 4 years after the fall of Jerusalem and 2 years after the mourning began for Gedaliah, then the Jews still should have been in exile 70 years after the fall of Jerusalem and 70 years after the mourning for Gedaliah began, with two years still to go. That is precisely the reference and circumstance of Zechariah 1 and 7. The Jews are still in exile at the end of 70 years after they began mourning for Gedaliah. But the 70 years of exile of the last deportees was not yet fulfilled and that is when the 70 years of Jeremiah would be fulfilled.
Therefore, we learn from this comparison that this is during the reign of Darius the Mede. The Bible says that when Babylon was conquered by the Medes and Persians, that Darius became king. It does not say the Jews were released in the first of Darius but the 1st of Cyrus. Nor does the Bible combine the rulership of Darius the Mede and Cyrus but when spoken of together, Cyrus follows Dairus the Mede as king. Note Daniel 6...
28 And as for this Daniel, he prospered in the kingdom of Da·ri´us and in the kingdom of Cyrus the Persian.
Again, the kingdoms are separate and the "kingdom of Darius" comes before the "kingdom of Cyrus."
Thus Zechariah proves that Darius the Mede ruled for six whole years before Cyrus began his rule.
But what about the end of the 70 years? Wasn't that supposed to end when Babylon was conquered? No! It was not supposed to end when the royalty of the Medes (Darius the Mede) began to rule but specifically when the royalty of Persia began to rule. That is a critical distinction. Note 2 Chronicles 36 in this regard:
20 Furthermore, he carried off those remaining from the sword captive to Babylon, and they came to be servants to him and his sons until the royalty of Persia began to reign; 21 to fulfill Jehovah’s word by the mouth of Jeremiah, until the land had paid off its sabbaths. All the days of lying desolated it kept sabbath, to fulfill seventy years.
The royalty of Persia is not the royalty of the Medes. Thus it was not until Cyrus comes to the throne, after a 6-year rule by Darius the Mede that the Jews are let go?
But what about the punishment of the kings of Babylon after 70 years? It means just that....
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,
Thus the overthrow of Babylon by the Medes did not result in the fulfillment of this prophecy if the king of Babylon was not to be punished until after the 70 years ended. Further, as the above verse in 2 Chronicles indicates, the Jews were to serve Nebuchadnezzar and "his sons" until the royalty of Persia began to reign. This little detail is thus fulfilled at the end of seventy years because of two historical technicalities regarding the king of Babylon:
1) Belshazzar was only a co-ruler, second in command when Babylon was overthrown. The #1 ruling king, Nabonidus, was still at large elsewhere in Borsippa focussing on the worship of his Moon God. Thus Darius the Mede was ruling over Babylon as second-in-command to Nabnoidus during these six years. But how could he do that?
2) Because Darius the Mede himself was the grandson of Nebuchadnezzar through a state marriage between the Medes and the Babylonians who had joined an alliance together during the time of Nabopolassar to help conquer Nineveh. So though he was the king of the Medes, he was a legitimate heir to the throne as a son of Nebuchadnezzar. All he did was killed his cousin, Belshazzar. That's probably why it was basically "business as usual" at Babylon and all the magistrates gladly accepted Darius the Mede as their king after Belshazzar was killed, because he was still a legitimate Babylonian king, though half-Mede.
But when Cyrus came to the throne, that ended the official Babylonian dynasty of kings and Nabonidus was put under house arrest by Cyrus, who was not recognized as the new, absolute king of all of Persia. Every other "king" in the empire had to relinquish that former title and become "governor". There would be only one king in the land.
So that's how it is all fulfilled. It's a little technical as far as details, but once everything is in place everything makes sense.
Another challenge to NB Timeline Revision: But now you have another critical problem. All the debate over when the 70 years of Jeremiah would begin and end pales to Zechariah's references showing the Jews still in exile 70 years after the fall of Jerusalem in the 2nd year of Darius. That confirms there was at least 70 years from the time Jerusalem fell until the 2nd of Darius and the Jews were still in exile! So you can claim the 70 years of Jeremiah began anytime you wanted, this is a historical reference (versus prophetic interpretation) that the rule of Darius the Mede was 6 years long before Cyrus came to the throne and that 74 years passed after the fall of Jerusalem before the Jews returned in the first of Cyrus. This is confirmed by Josephus who likewise says the 70 years began with the last deportation, meaning the Jews would return 74 years after the fall of Jerusalem. So you have perfect harmony between Josephus and the Bible, and yet another Biblical challenge to the NB chronology.
As far as the NB chronology goes, of course, they claim there was no 6-year rule by Darius the Mede! Others have said he is co-ruler with Cyrus for 2 years then disappeared off the scene or something. Others claim he was an imaginary character.
But in fact, when the NB Period was reduced in order to add years to the Persian Period, it was convenient to eliminate the entire rule of Darius the Mede because it was well-known that he and Cyrus together conquered Babylon. A total of 26 years were removed from the NB Period which is why the Bible's NB Period is 26 years longer than that of the revised chronology. The revision removed 2 years from Nebuchadnezzar II, 16 years from Evil-Merodach, 2 years from Nabonidus and the entire 6-year rule of "Darius the Mede" whom the Bible shows ruled 6 years.
Fortunately, though, the distorted history found in the Cyrus Cylinder and loose references in Herodotus do not ignore the importance of Darius the Mede historically, they just don't show him ruling those six years. What they did was to split up his identity into two different characters (as was common in revisionism documents back them). Thus Darius the Mede who conquered Babylon with Cyrus and ruled six years and then divided up the kingdom into districts, is divided historically over two characters, "Ugbaru" and "Gubaru". One of them conquers Babylon with Cyrus then dies shortly thereafter, Then another suddenly appears as "governor" and divides up the kingdom and rules for 14 years.
From this distorted reference, we correct the 14 years of "governorship" to include 6 years as king, in which case Darius the Mede continued to rule over Babylon as the "governor" of Cyrus until he died 8 years later. Cyrus would die the following year. But it is well-known that Cambyses, the son of Cyrus, began his co-rulership with Cyrus one year before his death and began that co-rulership from Babylon! Therefore, the co-rulership of Cambyses, dove-tails nicely with the death of governor Darius the Mede after 8 years as governor.
So the history of Darius the Mede survives, what he did generally or specifically, only the rulership for 6 years before Cyrus became king is distorted specifically. Even so, it is not difficult to backtrack by comparing several references with the Bible to reconstruct the likely original chronology.
Still believe the Bible is wrong and the Persians didn't revise their chronology? It's a JOKE to the well-informed and those who trust the Bible. But clearly there is no going back. At some point you must seriously consider revisionism of the Babylonian chronology and what evidence there is of such revisionism. When you do the VAT4956 ends all doubt since it contains double-dating to both 568BCE and 511BCE for year 37 of Nebuchadnezzar. The only reason for the "diary" was to hide the true chronology. I only mention this since it harmonizes with the 74-year interval established by Josephus. Year 37 in 511BCE dates the last deportation in year 525BCE. 70 years after 525BCE is 455BCE, the date the 70 weeks prophecy must match a time when the "word goes forth to rebuild Jerusalem" six months from Passover, which means the fall. The building began in the 7th month of the 1st year of Cyrus. The VAT4956 double-dating is another indicator of the revisionism but also a confirmation of the original chronology.
One last point, The 20 years of Cyurs: As a final note in this part of the history, please note that Cyrus ruled for 20 years over Persia before becoming king at Babylon. Per the popular secular history, these 20 years are recognized from 559 to 539BCE. When we apply this to his rule beginning in 455BCE, Cyrus would have begun his rule over Persian in 475BCE. That is the year he overthrew Astyages in the 6th year of Nabonidus. Thus we can date the 1st of Nabonidus 5 years earlier in 480BCE. Two eclipse references related to the dating of Nabonidus also date his first year in 480BCE (The Nabon 18, and the famous Thales eclipse mentioned by Herodotus: http://www.jehovahs-witness.com/10/96106/1.ashx )
SUMMARY: There are three periods of 70 years, each 2 years apart. One 70 years from the fall of Jerusalem (year 19), one 70 years from the beginning of the mourning for Gedaliah in the 7th month (year 21), and one 70 years from the last deportation (year 23). The last 70 years fulfills the Jeremiah prophecy that officially ends the Neo-Babylonian Period. The Jews were still in exile during a 7-year rule of Darius the Mede who was the grandson of Nebuchadnezzar and thus still a Babylonian king though Nabonidus was still at large. When Cyrus the "Persian" came to the throne, it officially ended the NB empire.