Tishrei was the first month until the exodus.
1And the LORD spake unto Moses and Aaron in the land of Egypt saying,
2This month shall be unto you the beginning of months: it shall be the first month of the year to you.
3Speak ye unto all the congregation of Israel, saying, In the tenth day of this month they shall take to them every man a lamb, according to the house of their fathers, a lamb for an house
So Noahs flood was truly in Cheshvan which is usually in Oct/Nov on our calendar because it happened before the exodus.