Albert, Bernard and Cheryl birthday puzzle

Albert and Bernard just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.

                                   May 15                   May 16            May 19
                                   June 17                  June 18
                                   July 14                   July 16
                                  August 14              August 15        August 17


Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively.


1.Albert:        I don't know when Cheryl's birthday is, but I know that Bernard does not know too.
2.Bernard:     At first I don't know Cheryl's birthday is, but I know now.
3.Albert:       Then I also know when Cheryl's birthday is.

So when is Cheryl's birthday?




Solution