No it's a period of time.
Matthew 24:29 And immediately after the tribulation of those days, the sun shall be darkened, and the moon shall not give her light, and the stars shall fall from the heaven, and the powers of the heavens shall be shaken;
That's multiple days. In Daniel's prophecy we see that the time left of the 490 years is seven years (one week of years). All of the events of Revelation occur during that seven year period.
>>Secondly, “is come” doesn’t mean the same thing as “has come”.<<
The Greek has past or present tense "has come". It is not a future anticipated event.
>>The future tense of this phrase is confirmed by the next phrase, also in the future tense: “who shall be able to stand?”<
That statement doesn't stand the scrutiny of the rest of scripture. Here is the literal word for word from the Greek:
"because come did the great day of His anger, and who is able to stand?
It says " come did" and "is able". Not will come or will be able.
“That’s multiple days.”
Yes, but what days are being referred to? It says the “immediately after the tribulation of those days”, so days refers to the tribulation, not the day of wrath, or day of the Lord as it is referred to in other prophecies.
In fact, the verse you cite is the best proof that the day of wrath cannot possibly occur during the tribulation. The same events described in that verse (darkening of the sun and moon, stars falling from heaven, and the heavens being disturbed) match the events described in Rev 6:12-17 that you used as your reference for the “day of wrath”, and this verse in Matthew clearly places these events after the tribulation!
“The Greek has past or present tense “has come”. It is not a future anticipated event.”
Well, I will take your word for that. If it’s true, then Rev 6 must be describing events after the tribulation, or it would be in conflict with Matthew 24.