code: public ProjectCalendar getEffectiveCalendar()
     {
        ProjectCalendar result = getCalendar();
        if (result == null)
        {
           result = getParentFile().getDefaultCalendar();
        }
        return result;
     }
docs: Retrieve the effective calendar for this task. If the task does not have
  a specific calendar associated with it, fall back to using the default calendar
  for the project.
  
  @return ProjectCalendar instance