Matthew and Mark both describe, almost verbatim, distress (Koine: thlipsis--pressure, and the internal stress that results) unique in that it was never equaled before nor will it be equaled after. So, the envisioned tribulation will surpass Noah’s flood according to this description, and that wiped out just about everything and everyone. Anything globally significant before WWII and the Holocaust, as well as those occurrences themselves, will also be surpassed on the same basis. The siege of Jerusalem in 66-70 CE isn't even in the ballpark by such a description--not in terms of severity, or scope, let alone in fulfilling cited prophecy.
Though Luke describes the circumstance by a different word (ananke), it's meaning (distress caused by external conditions) is roughly equivalent. Luke and Matthew do agree on describing that distress as great (megale). Although many events experienced in history up to the present could be described as great, nothing matches the depth and breadth of some of the events described in the Apocalypse. It seems to me that Jesus was referring to that unique level of things when he described the coming tribulation.
For the sake of the elect, the days of this tribulation are curtailed, which may explain the day variation of Daniel 12:11-12. Who might these elect be? Generally, the word signifies the chosen of God, and from the context, it can be assumed that the reference here includes Jews in Judea and Jerusalem looking for the Messiah. If others were meant to be encompassed by the term, we're certainly not told that in this discourse. Regardless, it's comforting to know that God has an agenda concerning time which has the aim of ending time without ending the elect.
Those aching to see the Messiah return, especially in the midst of such severe stress, could be susceptible to counterfeits. However, when Christ returns he won’t be slipping into town quietly, in a fashion that could be missed--not even by those not anticipating his return. Anything less than a cosmic, earth shaking event can't be the Messiah's return. Thankfully, it will not possible for the elect to be duped, which I suspect will owe much to the ministry of the Two Witnesses mentioned in Revelation 11:1-13.