The great tribulation occurs before he returns
Mtt 24:
15 “Therefore when you see the ‘abomination of desolation,’ spoken of by Daniel the prophet, standing in the holy place” (whoever reads, let him understand), 16 “then let those who are in Judea flee to the mountains. 17 Let him who is on the housetop not go down to take anything out of his house. 18 And let him who is in the field not go back to get his clothes. 19 But woe to those who are pregnant and to those who are nursing babies in those days! 20 And pray that your flight may not be in winter or on the Sabbath. 21 For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be. 22 And unless those days were shortened, no flesh would be saved; but for the elect’s sake those days will be shortened.
Christ returns AFTER this even, That is how he shortens those days.. or all life would be destroyed..