If date is == 0000-00-00 question

I have this code on my controller which get the users date of birth. It should show nothing if the date is 0000-00-00 from database but for some reason if not set then shows 01-01-1970

How can I make sure that if the date says 0000-00-00 on database then it will go to this $data['dob'] = '';

PHP Code:

if ($this->input->post('dob')) {
    $data['dob'] = $this->input->post('dob');
} elseif (!empty($user_info)) {
    $data['dob'] = date("d-m-Y", strtotime($user_info['dob']));
} else {
    $data['dob'] = '';
} 


There's only one rule - please don't tell anyone to go and read the manual.  Sometimes the manual just SUCKS!