code: public void add(ReadableDuration duration) {
          if (duration != null) {
              add(new Period(duration.getMillis(), getPeriodType()));
          }
      }
docs: Adds a duration to this one by dividing the duration into
  fields and calling {@link #add(ReadablePeriod)}.
  
  @param duration  the duration to add, null means add nothing
  @throws ArithmeticException if the addition exceeds the capacity of the period