code: public Period plusDays(int days) {
          if (days == 0) {
              return this;
          }
          int[] values = getValues();  // cloned
          getPeriodType().addIndexedField(this, PeriodType.DAY_INDEX, values, days);
          return new Period(values, getPeriodType());
      }
docs: Returns a new period plus the specified number of days added.
  <p>
  This period instance is immutable and unaffected by this method call.
  
  @param days  the amount of days to add, may be negative
  @return the new period plus the increased days
  @throws UnsupportedOperationException if the field is not supported