Albert and Bernard are trying to deduce Cheryl's birthday from a list of 10 dates. Cheryl tells Albert only the month, and Bernard only the day. Albert says: 'I don't know when her birthday is, but I know Bernard doesn't know either.' Bernard then says: 'At first I didn't know, but now I know.' Albert replies: 'Then I also know.' If the options are May 15, May 16, May 19, June 17, June 18, July 14, July 16, August 14, August 15, or August 17, when is her birthday?