The "Great Tribulation" is actually defined by Jesus in
Luke 21 as the punishment of the Jewish People in the present age. I don't need to quote it for you. I quote it all the time, and it is simply ignored by most. They don't want to believe what Jesus explicitly said.
Luke 21:19-24 KJV
19) In your patience possess ye your souls.
20) And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh.
21) Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto.
22) For these be the days of vengeance, that all things which are written may be fulfilled.
23) But woe unto them that are with child, and to them that give suck, in those days! for there shall be great distress in the land, and wrath upon this people.
24) And they shall fall by the edge of the sword, and shall be led away captive into all nations: and Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled.
Matthew 24:15-29 KJV
15) When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand)
16) Then let them which be in Judaea flee into the mountains:
17) Let him which is on the housetop not come down to take any thing out of his house:
18) Neither let him which is in the field return back to take his clothes.
19) And woe unto them that are with child, and to them that give suck in those days!
20) But pray ye that your flight be not in the winter, neither on the sabbath day:
21) For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
22) And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened.
23) Then if any man shall say unto you, Lo, here is Christ, or there; believe it not.
24) For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
25) Behold, I have told you before.
26) Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not.
27) For as the lightning cometh out of the east, and shineth even unto the west; so shall also the coming of the Son of man be.
28) For wheresoever the carcase is, there will the eagles be gathered together.
29) 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:
These are two different prophecies. Luke speaks of the destruction of Jerusalem and the scattering of the Jews, and the result being captivity among the nations. Matthew speaks of the Abomination of Desolation, and the greatest affliction of all time, and the result being the near destruction of humanity, ended when Jesus returns.
Much love!