code: public Instant withDurationAdded(ReadableDuration durationToAdd, int scalar) {
          if (durationToAdd == null || scalar == 0) {
              return this;
          }
          return withDurationAdded(durationToAdd.getMillis(), scalar);
      }
docs: Gets a copy of this instant with the specified duration added.
  <p>
  If the addition is zero, then <code>this</code> 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 a copy of this instant with the duration added
  @throws ArithmeticException if the new instant exceeds the capacity of a long