The original title is: A and B ask about C's birthday. C gave them 10 possible dates: May 15, May 16, May 19, June 17, June 18 and July/kloc-0. Results: A said: I don't know when is C's birthday. But I'm sure b doesn't know either. B said: I didn't know what C's birthday was at first. But I know now. A said, then I know which birthday C is now.

Answer:

"A said: I don't know which birthday C is", which means that the month C told A alone has at least two dates, which cannot be ruled out;

"But I'm sure B doesn't know either." It means that the day when C told B alone exists for at least two months, so May 19 and June 18 are excluded.

Because A doesn't know the answer, 6. 17 is also excluded.

Now the candidate answer is:

5/ 15 5/ 16

7/ 14 7/ 16

8/ 14 8/ 15 8/ 17

After A tells B, B says it knows, which means B's date is the only candidate answer, so it can only be 8/ 17.