Just a thought:
Revelation chapters 2 & 3 show Christ's warnings to the Christian congregations - letting them know that he sees what corrupt thinking some of them have.
At Revelation 2:22 he says, "Look! I am about to throw her into a sickbed, and those committing adultery with her into great tribulation, unless they repent of her deeds." Coupled with 1st Peter 4:17, that "the judgement starts with the house of God", Revelation 7:14 makes it clear that the Great Tribulation survivors are those Christians who have maintained their Christian faithfulness during Christ's testing.
So if Christ's "Great Tribulation" is the one and only "Great Tribulation", then how on earth could any feeble-minded individual/s claim to know when Christ is going to judge God's household of Christianity? A bit presumptious!! (never stopped them before, has it?)