Tricky question

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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.



Albert: I don't know when Cheryl's birthday is,  but i know that Bernard does not know too.

Bernard:At first I don't now when Cheryl's birthday is,but i know now.

Albert: Then i also know when Cheryl's birthday is.


So when  is Cheryl's birthday?                     

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