How do We Know a Month is 30 Days in the Bible?

"In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened" (Gen. 7.11).

"And the waters prevailed upon the earth an hundred and fifty days" (v.24).

"And the waters returned from off the earth continually: and after the end of the hundred and fifty days the waters were abated. And the ark rested in the seventh month, on the seventeenth day of the month, upon the mountains of Ararat" (Gen. 8.3-4).

150 / (7 - 2) = 30