code: public Duration getFinishSlack()
     {
        Duration finishSlack = (Duration) getCachedValue(TaskField.FINISH_SLACK);
        if (finishSlack == null)
        {
           Duration duration = getDuration();
           if (duration != null)
           {
              finishSlack = DateHelper.getVariance(this, getEarlyFinish(), getLateFinish(), duration.getUnits());
              set(TaskField.FINISH_SLACK, finishSlack);
           }
        }
        return (finishSlack);
     }
docs: Retrieve the finish slack.
  
  @return finish slack