Validate date of birth(date,moth and year)

if(preg_match('/^(0?[1-9]|1[0-2])\/(0?[1-9]|[12][0-9]|3[01])\/((19|20)\d{2})$/', $_POST['start_date']) == 0)
echo 'The given date is not in the format(mm/dd/yyyy)