Hi Magnum,
It is my understanding that the WTS starts its "70 Years" [of desolations] at the same moment that it starts its "2,520 Years" [of Gentile Rule]. These are the periods I meant and I apologise for my imprecision.
The question in my mind concerned the Society's starting event of that period: (1) When Jerusalem was destroyed [but that was in the 5th month]; (2) When Gedaliah was murdered; (3) When the Jews started on their trek to Egypt; (4) When Zedekiah was captured; (5) When Jews left the soil of Judah [ostensibly leaving the land depopulated.
The following is all I was able to readily locate.
Doug
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The lowly people that King Nebuchadnezzar left behind in the land of Judah had a governor appointed by him over them, namely, Gedaliah. However, he was killed in the seventh month (Tishri), and then the remaining Jews fled down to Egypt out of fear of Babylon, but only to have the hand of the king of Babylon reach them down there later on. In this way the land was left desolate in the seventh month, without man or beast, as Jeremiah had foretold. (Babylon the Great Has Fallen!, pages 166-167)
Jehovah's purpose to have Jerusalem and the land of Judah emptied, desolated of both man and domestic animal. (Babylon the Great Has Fallen!, page 121)
The beginning of the seventy years of Judah's desolation had yet eleven years to wait (Ezekiel 1:1-3) They began after the last king, Zedekiah the uncle of Jehoiachin, was dethroned and when the land of Judah was left desolate. (Babylon the Great Has Fallen!, page 138)
“The cities of Judah I shall make a desolate waste without an inhabitant."—Jeremiah 34:8-22. (Babylon the Great Has Fallen!, page 153)
By the flight of the faithless, disobedient Jews down to Egypt the land of Judah was left desolate, without human inhabitant and domestic animals. This proved Jehovah's prophecy by Jeremiah true. It occurred toward the middle of the seventh month, Tishri or Ethanim (September-October), which would be near October 1, 607 B.C. (Babylon the Great Has Fallen!, page 163)
Like the forty-ninth year of the cycle of sabbaths, the Jubilee year was to be a sabbath year for the God-given land, and so a Jubilee sabbath of the land began in the seventh month, Tishri. (Leviticus 25:8-22) During that month, as the fearful Jews needlessly fled down to Egypt and left the land of Judah utterly desolate and without human inhabitant, a place to be shunned by passersby, the land must have heaved a sigh of relief, as it were. Now it began to enjoy an uninterrupted run of sabbath years in compensation for all the Sabbath years that the disobedient Israelites had failed to keep. How many years of sabbath rest was the land to enjoy? Figuratively, a perfect number of years—seventy. (2 Chronicles 36:17-23; compare also Daniel 9:1, 2). (Babylon the Great Has Fallen!, pages 163-164)
Those Gentile Times, those "appointed times of the nations," would end 2,520 years from near the middle of the seventh lunar month (Tishri) of 607 B.C. (Babylon the Great Has Fallen!, page 180)
Jeremiah chapter 52 describes the momentous events of the siege of Jerusalem, the Babylonian breakthrough, and the capture of King Zedekiah in 607 B.C.E. Then, as verse 12 states, “in the fifth month, on the tenth day,” that is, the tenth day of Ab (corresponding to parts of July and August), the Babylonians burned the temple and the city. However, this was not yet the starting point of the “seventy years.” Some vestige of Jewish sovereignty still remained in the person of Gedaliah, whom the king of Babylon had appointed as governor of the remaining Jewish settlements. “In the seventh month,” Gedaliah and some others were assassinated, so that the remaining Jews fled in fear to Egypt. Then only, from about October 1, 607 B.C.E., was the land in the complete sense “lying desolated … to fulfill seventy years.”—2 Ki. 25:22-26; 2 Chron. 36:20, 21. (All Scripture is Inspired of God and Beneficial, page 285)
The 70 years were to be a period when the land of Judah and Jerusalem would enjoy “sabbath rests.” This meant that the land would not be cultivated—there would be no sowing of seed or pruning of vineyards. (Leviticus 25:1-5, NIV) … When did the land of Judah become desolated and unworked? … When did the 70 years commence? Certainly not following the first time that Nebuchadnezzar laid siege to Jerusalem. Why not? Although at that time Nebuchadnezzar took many captives from Jerusalem to Babylon, he left others behind in the land. …
[The Babylonians] razed the city, including its sacred temple, and they took many of its inhabitants captive to Babylon. Within two months, “all the people [who had been left behind in the land (added by the WTS)] from the least to the greatest, together with the army officers, fled to Egypt for fear of the Babylonians.” (2 Kings 25:25, 26, NIV) Only then, in the seventh Jewish month, Tishri (September/October), of that year could it be said that the land, now desolate and unworked, began to enjoy its Sabbath rest. To the Jewish refugees in Egypt, God said through Jeremiah: “You have seen all the disaster that I brought upon Jerusalem and upon all the cities of Judah. Behold, this day they are a desolation, and no one dwells in them.” (Jeremiah 44:1, 2, English Standard Version) So this event evidently marked the starting point of the 70 years. (The Watchtower, October 1, 2011, pages 27-28)
Jehovah allowed the Babylonians to conquer his people, destroy Jerusalem and its temple, remove Zedekiah from “the throne of the kingship of Jehovah” and take the Jews into Babylonian exile. Events that followed “in the seventh month” led the few Jews who had remained in the land to flee to Egypt, so that Judah then lay completely desolate. (Kingdom Comes, page 136)
Historians calculate that Babylon fell in early October of the year 539 B.C.E. (Kingdom Comes, page 136)
October, 537 B.C.E., which date therefore marks the completion of the foretold 70 years of desolation. That historical information is important to us in determining the beginning of “the appointed times of the nations.” Since the 70 years of desolation for Judah and Jerusalem ended in 537 B.C.E., they began in 607 B.C.E. That would be the year when Zedekiah ceased to sit upon the “throne of the kingship of Jehovah” in Jerusalem. It therefore marks also the date for the beginning of the Gentile Times. (Kingdom Comes, page 136)
The prophet Jeremiah predicted that the Babylonians would destroy Jerusalem and make the city and land a desolation. (Jeremiah 25:8, 9) He added: “And all this land must become a devastated place, an object of astonishment, and these nations will have to serve the king of Babylon seventy years.” (Jeremiah 25:11) The 70 years expired when Cyrus the Great, in his first year, released the Jews and they returned to their homeland. (2 Chronicles 36:17-23) We believe that the most direct reading of Jeremiah 25:11 and other texts is that the 70 years would date from when the Babylonians destroyed Jerusalem and left the land of Judah desolate.—Jeremiah 52:12-15, 24-27; 36:29-31. (Kingdom Comes, pages 187-188)
The land of Judah was to keep a “sabbath, to fulfill seventy years.” (2 Chronicles 36:21) How? By lying as a “desolate waste without man and domestic animal”. … Those seventy years of utter desolation of the land of Judah and Jerusalem without man and domestic animal. (Paradise Restored, page 132)
The seventy years of unbroken captivity to Babylon did not begin until 607 B.C.E., in the month Ethanim, when the land was left completely desolate when its remaining inhabitants went down to Egypt. Then the Jews as a nation went into exile at Babylon, without a king at Jerusalem. This exile was for an uninterrupted period of seventy years. (The Watchtower, December 1, 1964, page 735)