Sometimes scripture can become confusing because of the way a time frame is written. In verse 1, Joel wrote, “At that time, when I restore the prosperity of Judah and Jerusalem, says the Lord. . .” What time is God referring to, the time 70 CE when people were escaping Jerusalem before it was destroyed by Titus and the Roman army or is He referring to the time when “And I will pour out My Spirit. . .”? I think it is the latter.
We must remember that God sees the beginning to the end where we think in much shorter time cycles. In verses 3-6, God was referring to the time of the first century and the evil that happened to the people of Israel. In verses 7-16, God referred to the retribution He planned for the unbelieving nations who hate Israel. This will be (I think) when Christ returns to set up His kingdom on earth forever.
We read in verses 17-21 what a wonderful place Jerusalem will be. “For I will avenge the blood of My people; I will not clear their oppressors of guilt. For My home is in Jerusalem with My people.” (Verse 21).