Matthew 24:29-31
"Immediately after the tribulation of those days, the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken.
Then the sign of the Son of man will appear in heaven, and all the tribes of the earth will beat themselves in grief, and they will see the Son of man coming on the clouds of heaven with power and great glory.
And he will send out his angels with a great trumpet sound, and they will gather his chosen ones together from the four winds, from one extremity of the heavens to their other extremity."
Mark 13:24-27
"But in those days, after that tribulation, the sun will be darkened, and the moon will not give its light, and the stars will be falling out of heaven, and the powers that are in the heavens will be shaken.
And then they will see the Son of man coming in the clouds with great power and glory.
And then he will send out the angels and will gather his chosen ones together from the four winds, from earth’s extremity to heaven’s extremity."
After the attack on Jerusalem, the Christians weren't gathered together - they were scattered. Nobody saw angels and nobody saw Jesus coming on the clouds with power and great glory.
The time of "great distress" hasn't happened yet. Everybody will know when it happens. No one will have to guess. It'll be so bad, people will think no Christians will survive. That's why Jesus said this in Matthew 24:
"Truly I say to you that this generation will by no means pass away until all these things happen. Heaven and earth will pass away, but my words will by no means pass away."
...and this in Mark 13...
"Truly I say to you that this generation will by no means pass away until all these things happen. Heaven and earth will pass away, but my words will by no means pass away."
The leaders of the religious institutions (like the "heavens") will pass-away/be-removed and the organizations/bodies of the religious institutions (like the "earth" of those organizations) will be dissolved, but the "generation" of Christians who see the tribulation begin will survive to the end of the tribulation, because the appointed times of the nations to attack/rule-as-UN-world-power during the tribulation is only 3 1/2 years, and Jesus will help us get through that "wilderness" time.
He wasn't saying the "this generation" thing as some kind of way to calculate a timeframe. He was saying it so people wouldn't be scared that they wouldn't survive the tribulation.
The trialsome time period will have a definite ending and a definite end.
"But as these things start to occur, stand up straight and lift up your heads, because your deliverance is getting near."