code: public void setInto(ReadWritablePeriod period, Object object, Chronology chrono) {
          String str = (String) object;
          PeriodFormatter parser = ISOPeriodFormat.standard();
          period.clear();
          int pos = parser.parseInto(period, str, 0);
          if (pos < str.length()) {
              if (pos < 0) {
                  // Parse again to get a better exception thrown.
                  parser.withParseType(period.getPeriodType()).parseMutablePeriod(str);
              }
              throw new IllegalArgumentException("Invalid format: \"" + str + '"');
          }
      }
docs: Extracts duration values from an object of this converter's type, and
  sets them into the given ReadWritableDuration.
  
  @param period  period to get modified
  @param object  the String to convert, must not be null
  @param chrono  the chronology to use
  @return the millisecond duration
  @throws ClassCastException if the object is invalid