“And as it happened in the days of Noah, even so will it be also in the days of the Son of man.”
Here the ‘days of Noah' undoubtedly refer to the days prior to the day that Noah left the world and entered the Ark as the next verse makes clear. We would therefore expect the parallel phrase ‘the days of the Son of Man' to signify the days prior to the climactic events that happened to Him, the days that led up to Him too leaving the world. The future tense in the latter case need indicate no more than that the days of the Son of Man are not yet complete, and the climax is yet to come. It is especially that climax that is in the future.