The Launch Date Only Two VPs Could Deduce

A founder narrows her product launch to ten candidate dates: May 15, May 16, May 19, June 17, June 18, July 14, July 16, August 14, August 15, August 17. She privately tells VP Aiden only the month and VP Bella only the day number. Aiden announces: "I don't know the date, but I'm certain Bella doesn't know it either." Bella responds: "I didn't know at first, but now I do." Aiden then says: "Now I know it too." What is the launch date?