John Gill Commentary Luke 17:26

"And as it came to pass in the days of Noah, even so shall it be also in the days of the Son of man." — Luke 17:26 (ASV)

And as it was in the days of Noe
Whilst he was building the ark, and before he went into it; for this respects the days of Noah before the flood, and not after it; for he lived after the flood three hundred and fifty years, (Genesis 9:28)

so shall it be also in the days of the son of man ;
some time before, and at his coming in power, and great glory, to destroy the Jews, their nation, city, and temple; and as then, so it will be when he shall come in person, at the last day, to destroy the world: the times of Noah's flood, of Jerusalem's destruction, and of the end of the world, bear a great resemblance to each other:

And when the son of man comes in either of these senses, then will the kingdom of God come; or then will it appear that the Messiah is come, and has took to himself his great power, and reigns.