code: public Duration add( long duration,
                           TimeUnit unit ) {
          long durationInNanos = TimeUnit.NANOSECONDS.convert(duration, unit);
          return new Duration(this.durationInNanos + durationInNanos);
      }
docs: Add the supplied duration to this duration, and return the result.
  
  @param duration the duration to add to this object
  @param unit the unit of the duration being added; may not be null
  @return the total duration