According to Genesis, on what date did the Flood begin in Noah's Ark's story?

According to Genesis, the Flood began on the 17th day of the second month in Noah's six-hundredth year.

Genesis 7:11 gives the date while describing the opening of the great deep and the windows of heaven. The narrative uses Noah’s age as a chronological reference, stating that he was 600 years old when the Flood began.

This date is distinct from the forty days and forty nights of rain often remembered in summaries. Genesis mentions the rain lasting forty days and nights, but that is a duration, not the starting date. The waters also remained for a much longer period before the Ark came to rest and Noah eventually left it.

The text does not identify this month with a universally agreed modern calendar month. Biblical calendar systems and later interpretations differ in how dates are mapped onto modern calendars. The precise wording supplied by Genesis is therefore the safest answer: the 17th day of the second month.