code: public Date getStartDate()
     {
        Date result = (Date) getCachedValue(ProjectField.START_DATE);
        if (result == null)
        {
           result = getParentFile().getStartDate();
        }
        return (result);
     }
docs: Retrieves the project start date. If an explicit start date has not been
  set, this method calculates the start date by looking for
  the earliest task start date.
  
  @return project start date