docs: public DateMidnight withFieldAdded(DurationFieldType fieldType, int amount) {
          if (fieldType == null) {
              throw new IllegalArgumentException("Field must not be null");
          }
          if (amount == 0) {
              return this;
          }
          long instant = fieldType.getField(getChronology()).add(getMillis(), amount);
          return withMillis(instant);
      }