I believe that the "tribulation" mentioned at Matthew 24:21 is limited to Israel. After the tribulation is over then the signs will be seen in the sky (Mt.24:29) and then we see that after that there will be judgment coming on the whole earth:
"And there shall be signs in the sun, and in the moon, and in the stars; and upon the earth distress of nations, with perplexity; the sea and the waves roaring; Men's hearts failing them for fear, and for looking after those things which are coming on the earth: for the powers of heaven shall be shaken...For as a snare shall it come on all them that dwell on the face of the whole earth"(Lk.21":25-26;35).
So when we see the reference to the signs in the sky in the book of Revelation we know that the events spoken of after that refer to the events which will come on the whole earth and have nothing to do with the tribulation mentioned at Matthew 24:21:
"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; And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind. And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places. And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains; And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb" (Rev.6:12-16).
