Tim,
I believe that there are no contradictions in the Word of God. If there are contradictions - when we might as well toss it all away - and go with the philosophy - "eat, drink, and be merry, for tomorrow we die."
When I study passages of Scripture like Joel 2:31; Acts 2:19,20 and Rev. 6:12 - I can come to two conclusions:
1. Joel 2:31 and Acts 2:19,20 are not about the same events as Rev. 6:12;
2. Or Rev. 6:12 is God's commentary (further amplification) of the other two verses. (My preference)
Quote from MacArthur Study Bible - Introduction to Joel:
"A second issue confronting the interpreter is Peter's quotation from Joel 2:28-32 in Acts 2:16-21. Some have viewed the phenomena of Acts 2 and the destruction of Jerusalem in A.D. 70 as the fulfillment of the Joel passage, while others have reserved its fulfillment to the final Day of the Lord only -- but clearly Joel is referring to the final terrible Day of the Lord. The pouring out of the Holy Spirit at Pentecost not a fulfillment, but a preview and sample of the Spirit's power and work, to be released fully and finally in the Messiah's kingdom after the Day of the Lord. See note on Acts 2:16-21"
MacArthur - Acts 2:16-21:
"See Introduction to Joel: Interpretive Challenges; see note on Joel 2:28-32. Joel's prophecy will not be completely fulfilled until the millennial kingdom and the final judgment. But Peter, by using it, shows that Pentecost was a pre-fulfillment, a taste of what will happen in the millennial kingdom when the Spirit is poured out on all flesh."
MacArthur - Joel 2:28-32:
" 2:28 afterward. The abundance of material blesings would be followed by the outpouring of spiritual blessings. When coupled with the other temporal phrases within the passage ('in those days'[v.29] and 'before the coming of the great and awesome Day of the Lord' [v.31]), the term points to a Second Advent fulfillment time frame. all flesh. Since the context is 'your sons and your daughters,' 'all flesh' best refers to the house of Israel only. The nations are recipients of God's wrath, not the effusion of His Spirit (cf.3:2,9ff)."
"2:30,31 Before...day of the Lord. Unmistakable heavenly phenomena will signal the imminent arrival of God's wrath in the Day of the Lord (cf.v.10)."
"2:32 whoever calls. Quoted by Paul in Rom. 10:13. remnant. In spite of the nation's sin, God promised to fulfill His unconditional covenants (Noahic, Abrahamic, Davidic, and New). A future remnant of Jews will inherit God's promised blessings (cf.Is. 10:20-22; 11:11,16; Jer. 31:7; Mic 2:12; Zeph 3:13; Rom. 9:27)."