code: public DateTime withDurationAdded(long durationToAdd, int scalar) {
          if (durationToAdd == 0 || scalar == 0) {
              return this;
          }
          long instant = getChronology().add(getMillis(), durationToAdd, scalar);
          return withMillis(instant);
      }
docs: Returns a copy of this datetime 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
  @param scalar  the amount of times to add, such as -1 to subtract once
  @return a copy of this datetime with the duration added
  @throws ArithmeticException if the new datetime exceeds the capacity of a long