Exegesis:

kai kathōs egeneto … houtōs estai ‘and as it was … so it will be.’ Both parts of the comparison refer to a situation existing for some time and suddenly brought to an end. Hence egeneto is best understood as equivalent of ēn. kathōs and houtōs define two situations as similar.

en tais hēmerais Nōe ‘in the time of Noah,’ i.e. the time before he went into the ark.

en tais hēmerais tou huiou tou anthrōpou ‘in the days of the Son of man,’ best understood as analogous to hai hēmerai Nōe and as denoting the period, or time preceding the appearing of the Son of man.

Translation:

In the days of Noah, or, ‘in the (former) day/time of N.,’ ‘when N. lived.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.