code: public static final String roll(String isoString, String fmt, int field,
                                      boolean up) throws ParseException {
  
          Calendar cal = GregorianCalendar.getInstance();
          cal.setTime(stringToDate(isoString, fmt));
          cal.roll(field, up);
  
          return dateToString(cal.getTime(), fmt);
      }
docs: Time Field Rolling function. Rolls (up/down) a single unit of time on the
  given time field.
  
  @param isoString
  @param field
  the time field.
  @param up
  Indicates if rolling up or rolling down the field value.
  @param fmt
  use formating char's
  @exception ParseException
  if an unknown field value is given.