“150 days.” Scripture says the waters prevailed for 150 days. There are commentators who assert that the 150 days proves that there is something they call a “prophetic month” of 30 days. They claim that prophetic times in Scripture are calculated on a premised 30-day month. However, there is no traditional source for such a thing—it seems to be a concept built to accommodate their calculations. The Jews based their month from the moon, and there is no Jewish concept of a straightforward 30 day month. That concept is being read back into history, but is not a part of it. Some of the authors who try to defend the 30-day month use for their first proof that the flood year with the statement of the seventeenth of the second month, when the rains started, to the rain’s end, the seventeenth of the seventh month, is listed as 150 days. So, they say from seventeenth of Marcheshvan to the seventeenth of Nisan (of course, counting the first of the year being Tishri, as it was supposed to be in remote antiquity) is the 150 days, counting months as having 30 days. This would seem to be so, since counting that time as lunar months should come out to 147 days or so.
The first volume of “The Book of Genesis” under the series “Books of the Bible” published by the Judaica Press contains the Hebrew text, their own translation, and extensive commentaries taken from Rashi, Rambam, Eben Ezra, and others. What they say is that the “seventh month” means not the month on the calendar, but the seventh month, counting from when the rain started. Their calculation goes:
Kislev —three days (after the 40 days of rain beginning in the “second month” Marcheshvan, leaving three days in Kislev)
Tevet 29, Shevat 30, Adar 29, Nisan 30, Iyar 29, = 150
These are Jews commenting on their own Scripture and we would give them more than a little weight on this issue.