How long does it take to fly from Philadelphia pa to Seoul south Korea?

Flying Delta you first stop in Atlanta, GA. Philly to Atlanta is 2 to 2.5 hrs, depending on the speed & size of the plane. Atlanta to Inchon non-stop is 15 hours, for a total of 17.5 hours flying time. Include the layover in Atlanta, arriving at the airport 3 hours ahead, inevitable delays, and it's closer to 20 hours. South Korea is 13 hours ahead so you're flying into tomorrow and returning the day before. Bon Voyage!