Pretribulational John F. Walvoord writes:

“‘And I will shew wonders in the heavens and in the earth, blood, and fire, and pillars of smoke. The sun shall be turned into darkness, and the moon into blood, before the great and terrible day of the Lord come.’ What is meant here is not that the day of the Lord will begin after these wonders in heaven, but that it will come to its climax when the judgment is actually executed” (emphasis mine). Posttribulationism Today, Part IX: The Rapture and the Day of the Lord in 1 Thessalonians 5,” p. 7

Apparently for Walvoord, “before” means “after.” And 2 + 2 = 5
The reason why Walvoord interprets it this way is because his pretribulational system requires this. That is, in Walvoord’s mind, the Day of the Lord must start at the beginning of the 7 year period, so having the Day of the Lord occur after the cosmic disturbances would place the Day of the Lord for him much further into the 7 year period; hence, his blind spot when he comes to the explicit text of Joel 2:30-31.
Incidentally, it is interesting as a spectator watching the pretribulational vs. postribulational debate between Walvoord and Gundry in the 70’s and 80’s. Since pretribs place the starting point of the Day of the Lord at the beginning of the 7 year period, and (most) post-tribs place it at the end, both are two extreme errors. Prewrath solves this whole dilemma by recognizing that Scripture teaches that the Great Tribulation is cut short, which naturally allows for the Day of the Lord to unfold for the remaining part of the 7 year period. And it is the only view that can provide a non-strained reading of the cosmic disturbances.

So problematic is Joel 2:30-31 for pretribulationists, one professor has basically written an entire dissertation attempting to refute the prewrath interpretation of this passage! (Dr. Alan Cole, “A Critique of Prewrath Rapturism’s Terminus A Quo of the Day of the Lord” (Th.D. diss., Central Baptist Theological Seminary, 2004). By the way, I provide a response to his thesis in my forthcoming book.