For various reasons, some people don't know their birthdays. I recently met one family whose members did not know the month and day of their birth dates. When I asked a younger member about his birthday, he referred me to his parents. Unfortunately, his parents had no idea when he was born though they did manage to provide an estimated birth year. Moreover, they did not even remember their own birthdays. The parents would refer me to the grandparents who can only give vague answers like "he was born during the war" or "she was born in Easter week".
In all, not a single person in this household of eight even knew or remembered when he or she was born! The first thing that came to my mind was that this family might be aliens from space. But then again, technologically advanced aliens like the ones from sci-fi movies would have saved their birth dates in a chip to be accessed later.