code: public Duration getStartSlack()
     {
        Duration startSlack = (Duration) getCachedValue(TaskField.START_SLACK);
        if (startSlack == null)
        {
           Duration duration = getDuration();
           if (duration != null)
           {
              startSlack = DateHelper.getVariance(this, getEarlyStart(), getLateStart(), duration.getUnits());
              set(TaskField.START_SLACK, startSlack);
           }
        }
        return (startSlack);
     }
docs: Retrieve the start slack.
  
  @return start slack