docs: public static String getBirth(String idCard) {
  		final Integer len = idCard.length();
  		if (len < CHINA_ID_MIN_LENGTH) {
  			return null;
  		} else if (len == CHINA_ID_MIN_LENGTH) {
  			idCard = convert15To18(idCard);
  		}
  		return idCard.substring(6, 14);
  	}