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