On what date did the Flood begin in Genesis’s calendar?

Genesis says the Flood began on the seventeenth day of the second month. Genesis 7:11 dates the event to Noah’s six-hundredth year, on that specific day, when the fountains of the deep opened and the windows of heaven were opened.

This is a relative date within the calendar used by the biblical narrative, not a modern Gregorian date. Genesis does not identify the month with a universally agreed modern calendar month, so converting it to a contemporary date would add an assumption not supplied by the text.

The date is also different from later milestones in the story. Genesis records that the Ark came to rest in the seventh month, and that the mountain tops became visible in the tenth month. Those are stages after the Flood began. The seventeenth day of the second month is specifically the opening date given for the catastrophe.