There are a number of good arguments for dating Joel to the late ninth century.
1. Joel is grouped with the earliest prophets. Hosea, Amos, Jonah, and Micah are eighth century prophets. Nahum, Habakkuk, and Zephaniah are seventh century prophets. Haggai, Zechariah, and Malachi are post-exilic prophets. Thus, while the books are not grouped according to strict chronology, they are grouped chronologically, and this favors a pre-exilic date for Joel.
2. The nations that Joel mentions (“Egypt and Edom, Tyre, Sidon, Philistia, Greece and the Sabeans of Arabia”) and the ones he fails to mention (Assyria, Babylon, Persia) favor an early date. Furthermore, Amos mentions several of the same enemies as Joel as well as a locust plague, which may indicate that he was looking back to the same incidents that Joel prophesied about.
3. Leaving aside Obadiah and Joel, the earliest mention of the Day of Lord in the Minor Prophets occurs in Amos (dated to the eighth century). It would be somewhat puzzling for Amos to contain the first revelation on this topic because Amos 5:18 reveals that the Israelites have already twisted the Day of the Lord to apply only to Israel’s enemies and not to themselves. On the other hand, Robert Bell observes, If Obadiah and Joel preached their messages on the day of the Lord in the ninth century, then it is completely understandable that the eighth-century Amos found the term on the lips of carnal Israelites.”
4. Joel quoted Obadiah (which we have already dated to the mid-ninth century) and Amos (dated to the mid-eighth century) quotes Joel. Thus, Joel should be dated between Obadiah and Amos.
4.a. Joel 2:32 quotes Obadiah 17. The key indication that Joel is quoting Obadiah rather than the other way around is the inclusion in Joel of the phrase, “as Yhwh has said.” That phrase is indicating Joel’s reliance on Obadiah’s earlier prophecy.
4.b. Joel 3:16-17 quotes Amos 1:2. Amos is dealing with Israelites who are applying the Day of the Lord only to their enemies, and throughout the book Amos is warning that they too are liable to God’s Day of the Lord judgment. Amos 1:2 takes Joel’s eschatological roar of Yhwh from Zion against the nations and applies it to contemporary Jerusalem and against Israel.
 For a survey of six proposed dates spanning this date range, see Barker, ZECOT, 28-29; cf. Finley, WEC, 2.
 O. Palmer Robertson, Prophet of the Coming Day of the Lord, 10; Robertson, Christ of the Prophets, 244; Busenitz, MC, 32; Nass, CC, 25.
 Robertson, Prophet, 10-11; cf. Schmoller, Lange’s Commentaries, 3-4; Young, Introduction to the Old Testament, 248; Robertson, Christ of the Prophets, 244-45; Busenitz, MC, 33.
 Schmoller, 3-4.
 Bell, The Theological Messages of the Old Testament Books, 377-78; cf. Busenitz, MC, 33.
 Schmoller, 3-4; cf. Young, Introduction to the Old Testament, 248 (with regard to Amos quoting Joel); Archer, A Survey of Old Testament Introduction, 339 (with regard to Amos quoting Joel); Busenitz, MC, 34 (Busenitz places Joel in the reign of Jehoshaphat, which would predate Obadiah on my reckoning, but he rightly recognizes that Amos quotes Joel).
 cf. Niehaus, “Amos,” Minor Prophets, 338.