docs: public Duration toStandardDuration() {
          checkYearsAndMonths("Duration");
          long millis = getMillis();  // no overflow can happen, even with Integer.MAX_VALUEs
          millis += (((long) getSeconds()) * ((long) DateTimeConstants.MILLIS_PER_SECOND));
          millis += (((long) getMinutes()) * ((long) DateTimeConstants.MILLIS_PER_MINUTE));
          millis += (((long) getHours()) * ((long) DateTimeConstants.MILLIS_PER_HOUR));
          millis += (((long) getDays()) * ((long) DateTimeConstants.MILLIS_PER_DAY));
          millis += (((long) getWeeks()) * ((long) DateTimeConstants.MILLIS_PER_WEEK));
          return new Duration(millis);
      }