So the angel of Jehovah said: “O Jehovah of armies, how long will you withhold your mercy from Jerusalem and the cities of Judah, with whom you have been indignant these 70 years?” (Zechariah 1:12)
“Say to all the people of the land and to the priests, ‘When you fasted and wailed in the fifth month and in the seventh month for 70 years, did you really fast for me? (Zechariah 7:5)
In these verses, two seventy year periods are mentioned. The first is a period of God’s indignation against Jerusalem. The second, a period of fasting carried on by the Jews.
In the second time period, Jehovah refers to two fasts, one in the fifth month and the other in the seventh month. Later in Zechariah’s prophecy two additional fasts are mentioned, one in the fourth month and another in the tenth month. (Zech. 8:19)
With what are these fasts connected?
In the reference book Insight on the Scriptures, the Watchtower society says:
“Four Annual Fasts of the Jews. The Jews established many fasts, and at one time had four annual ones, evidently to mark the calamitous events associated with Jerusalem’s siege and desolation in the seventh century B.C.E. (Zec 8:19) The four annual fasts were: (1) “The fast of the fourth month” apparently commemorated the breaching of Jerusalem’s walls by the Babylonians on Tammuz 9, 607 B.C.E. (2Ki 25:2-4; Jer 52:5-7) (2) It was in the fifth Jewish month Ab that the temple was destroyed, and evidently “the fast of the fifth month” was held as a reminder of this event. (2Ki 25:8, 9; Jer 52:12, 13) (3) “The fast of the seventh month” was apparently held as a sad remembrance of Gedaliah’s death or of the complete desolation of the land following Gedaliah’s assassination when the remaining Jews, out of fear of the Babylonians, went down into Egypt. (2Ki 25:22-26) (4) “The fast of the tenth month” may have been associated with the exiled Jews already in Babylon receiving the sad news that Jerusalem had fallen (compare Eze 33:21), or it may have commemorated the commencement of Nebuchadnezzar’s successful siege against Jerusalem on the tenth day of that month, in 609 B.C.E.—2Ki 25:1; Jer 39:1; 52:4.
According to Josephus (The Jewish War, VI, 250, 268 [iv, 5, 8]), Herod’s temple was also burned by the Romans on the tenth day of the fifth month (70 C.E.), and Josephus makes note of the precise correspondency of this date with the burning of the first temple on the same day by the Babylonians. The fast in the seventh month was in memory of the murder of Gedaliah. (2 Kings 25:25; Jer. 41:1,2) This shows that the start of the 70 years of Zechariah’s prophecy began with the demolition of Jerusalem.
When certain Jews asked: “Shall I weep in the fifth month, practicing an abstinence, the way I have done these O how many years?” by means of Zechariah, Jehovah answered: “When you fasted . . . for seventy years, did you really fast to me, even me?” God showed that a real fast to him would have been accompanied by obedience and that what he required was truthfulness, judgment, peace, and a sincere heart. Then, instead of fasting mournfully and looking back into the past, they would be able to exult and rejoice in festal seasons with the blessings of restoration of true worship and ingathering of others to Jehovah’s service.—Zec 7:3-7; 8:16, 19, 23. (Insight on the Scriptures vol.1 p. 812)
The question is when did Zechariah record this vision?
And in the fourth year of King Da·riʹus, the word of Jehovah came to Zech·a·riʹah on the fourth day of the ninth month, that is, the month of Chisʹlev. (Zechariah 7:1)
When was the fourth year of King Darius?
Once again, Insight on the Scriptures states:
The last time indicator found in the book of Zechariah is the fourth day of Chislev in the fourth year of Darius’ reign (about December 1, 518 B.C.E.). (Insight on the Scriptures vol. 2 pg. 1225)
Hence, the second year of King Darius when Zechariah received his first vision would be 2 years earlier or 520/519 BCE. If Jehovah had denounced Jerusalem and the cities of Judah for 70 years up to that point as the angel indicated (Zech. 1:12), this would mean that his denunciation began in 590/589 BCE.
In the fourth year of Darius, (518/517 BCE) Zechariah indicates that the Jews had been fasting for 70 years. (Zech. 7:1-4)
According to Ezra, the temple was completed in the sixth year of Darius or 516/515 BCE. (Ezra 6:15) (See The Watchtower March 2022 pg. 16 par. 7 “Do You See What Zechariah Saw?”)
If indeed the 70 years of fasting of the Jews for the destruction of Jerusalem and its temple finally ceased in 518/517 BCE in the fourth year of Darius, then they began, not in 607 BCE as the Watchtower maintains, but in 588/587 BCE.