99.91% Can't Answer This!!!

Napoleon Bonaparte and Joseph Bonaparte had recently befriended Maria Luisa of Parma and they wanted to know her birthday. Maria Luisa then gave 10 choices:
May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, and August 17.
Maria Luisa then told Napoleon and Joseph separately about the month and date of her birthday in order.
Napoleon and Joseph then made statements:
Napoleon said: I don't know when Maria's birthday is, but I'm sure Joseph doesn't know it either. Joseph: At first, I didn't know Maria's birthday, but now I know.
Napoleon: Then I also know when Maria's birthday is.
So, when is Maria Luisa's birthday?
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