code: public static final String dateIncrease(String isoString, String fmt,
                                              int field, int amount) {
  
          try {
              Calendar cal = GregorianCalendar.getInstance();
              cal.setTime(stringToDate(isoString, fmt, true));
              cal.add(field, amount);
  
              return dateToString(cal.getTime(), fmt);
  
          } catch (Exception ex) {
              return null;
          }
      }
docs: 日期增加
  
  @param isoString
  日期字符串
  @param fmt
  格式
  @param field
  年/月/日 Calendar.YEAR/Calendar.MONTH/Calendar.DATE
  @param amount
  增加数量
  @return
  @throws ParseException