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.

Pretribulational teacher J. Dwight Pentecost asserts,

If the Day of the Lord did not begin until the second advent, since [the second advent] is preceded by signs, the Day of the Lord could not come as a “thief in the night,” unexpected, and unheralded, as it is said it will come in 1 Thessalonians 5:2. The only way this day could break unexpectedly upon the world is to have it begin immediately after the rapture of the church (Things to Come, p. 230).

According to Pentecost the Day of the Lord is not preceded by any signs. How can such a noted prophecy teacher in his day get it so wrong? The Scriptural evidence refutes such an assertion:
“The sunlight will be turned to darkness and the moon to the color of blood, before the day of the LORD comes- that great and terrible day!” (Joel 2:31)
“Look, I will send you Elijah the prophet before the great and terrible day of the LORD arrives.” (Mal 4:5)
“Now regarding the arrival of our Lord Jesus Christ and our being gathered to be with him, we ask you, brothers and sisters, (2) not to be easily shaken from your composure or disturbed by any kind of spirit or message or letter allegedly from us, to the effect that the day of the Lord is already here. (3) Let no one deceive you in any way. For that day will not arrive until the rebellion comes and the man of lawlessness is revealed, the son of destruction.” (2 Thess 2:1-3)
Yes, the Day of the Lord will come as a thief in the night, but not for believers. Perhaps, Pentecost should have read a little further:
“But you, brothers and sisters, are not in the darkness for the day to overtake you like a thief would.” (1 Thess 5:4)