code: public static String dateOffset(String initial, String offset) {
          Date date = convertFromAgmipDateString(initial);
          BigInteger iOffset;
          if (date == null) {
              // Invalid date
              return null;
          }
          GregorianCalendar cal = new GregorianCalendar();
          cal.setTime(date);
  
          try {
              iOffset = new BigInteger(offset);
              cal.add(GregorianCalendar.DAY_OF_MONTH, iOffset.intValue());
          } catch (Exception ex) {
              return null;
          }
          return convertToAgmipDateString(cal.getTime());
      }
docs: Offset an AgMIP standard date string (YYYYMMDD) by a set number of days.
  
  @param initial AgMIP standard date string
  @param offset number of days to offset (can be positive or negative
  integer)
  
  @return AgMIP standard date string of <code>initial + offset</code>