Genèse 7:12

And the rain came down upon the earth forty days and forty nights.

ויהי הגשם, as G’d had predicted so it came to pass; it kept raining non-stop for 40 days.

ארבעים יום, “forty days.” According to Rashi, the first day is not to be counted. (as the first night no rain had fallen) The Torah had made a point of writing in verse 11: ביום הזה נבקעו כל מעינות תהום, “on this day all the fountains of the deep broke open.” Accordingly the 40 days came to an end on the 28th day of Kislev.” This does not seem accurate, seeing that he himself wrote on verse 24, where the Torah wrote that the water level on earth kept increasing for a period of one hundred and fifty days which would mean that they kept increasing until the first day of Sivan where the Torah spoke about the seventh month as the day when the rains stopped descending. (compare his commentary on 8,4) We would therefore have to include the first day of the “forty” days as being part of that count. I believe therefore that the rains had started falling as soon as it was daylight on the seventeenth of the second month, as the words: ביום הזה in our verse do not prove anything about which part of the day it refers to. We find this expression in the Torah on numerous occasions when it clearly includes the preceding night. (The author quotes Genesis 2,2, and Leviticus 23,28 as examples to prove his point.) The author also cites Seder Olam, an ancient history text, as supporting his conclusion. He quotes that text verbatim: “on this day all the fissures in the earth broke open, and rain fell on the earth from the sky. This state of affairs continued until the 27th day of Kislev, but the level of water covering the surface of the earth continued to rise until the end of 150 days, that day being the first day of the month of Sivan.” In light of what the author of that text writes, we must include the first day of the “forty” days given in the Torah in our total.