Commentary for Genesis 7:11

בחדש השני IN THE SECOND MONTH — Rabbi Eliezer said, “This is the month Marcheshvan”; Rabbi Joshua said, “This is the month Eyar (Rosh Hashanah 11b).

בשנת שש מאות שנה לחיי נח, although Noach’s age had already been reported before, here the Torah adds further precise data about the month and the day of the month when the deluge commenced. בחודש השני, there is a disagreement between the scholars of the Talmud if the “second month” refers to a count based on the lunar year or the solar year. According to the view that the world was created in the month of Tishrey, (i.e. that Adam was created on the first of that moth), the month described here is the month of Marcheshvan, whereas according to the view that the universe was created in the month of Nissan, the deluge would have started in the month of Iyar. Rabbi Yonathan (Rosh Hashanah 11) decided that the correct version is that of Rabbi Eliezer who translated the verse בירח האתנים in Kings I 8,2 as the “ancient” month. It appears that this month was called איתנים, seeing it was the month (תשרי) in which the universe had been created. It was only after the Jewish people left Egypt, that they adopted the month of Nissan as the first month of their calendar year, (partially), as we know from Exodus 12,1-2. The verses there make sense only if the other nations had not used the month of Nissan as the first month of the calendar year.