code: 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);
  	}
docs: 根据身份编号获取生日，只支持15或18位身份证号码
  
  @param idCard 身份编号
  @return 生日(yyyyMMdd)