docs: public void setRounding(DateTimeField field, int mode) {
          if (field != null && (mode < ROUND_NONE || mode > ROUND_HALF_EVEN)) {
              throw new IllegalArgumentException("Illegal rounding mode: " + mode);
          }
          iRoundingField = (mode == ROUND_NONE ? null : field);
          iRoundingMode = (field == null ? ROUND_NONE : mode);
          setMillis(getMillis());
      }