code: public Duration withDurationAdded(ReadableDuration durationToAdd, int scalar) {
          if (durationToAdd == null || scalar == 0) {
              return this;
          }
          return withDurationAdded(durationToAdd.getMillis(), scalar);
      }
docs: Returns a new duration with this length plus that specified multiplied by the scalar.
  This instance is immutable and is not altered.
  <p>
  If the addition is zero, this instance is returned.
  
  @param durationToAdd  the duration to add to this one, null means zero
  @param scalar  the amount of times to add, such as -1 to subtract once
  @return the new duration instance