Darkened sun/moon to blood occurs TWICE???

[BillyShope]

An adjunct to the belief in a rapture prior to the beginning of Daniel's seventieth week is the teaching that the seventieth week is synonymous with the Day of the Lord. This leads to the inescapable conclusion that the darkening of the sun and the turning of the moon to blood must occur twice.

Prophecies of the first occurrence start in Isaiah and continue through Joel and Acts:

Behold, the day of the LORD cometh, cruel both with wrath and fierce anger, to lay the land desolate: and he shall destroy the sinners thereof out of it. For the stars of heaven and the constellations thereof shall not give their light: the sun shall be darkened in his going forth, and the moon shall not cause her light to shine.
Isaiah 13:9,10

The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the LORD come.
Joel 2:31

Peter quotes Joel:

The sun shall be turned into darkness, and the moon into blood, before that great and notable day of the Lord come:
Acts 2:20

The second occurrence is prophesied in Revelation:

And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;
Revelation 6:12

This second occurrence is also referenced in the Olivet Discourse:

Immediately after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
Matthew 24:29

Joel makes it very clear that an occurrence precedes the seventieth week (the Day of the Lord), while the Matthew and Revelation passages indicate a second occurrence after the appearance of  Antichrist and the associated tribulation.

Is this understanding correct?