code: private BigInteger getTaskCalendarID(Task mpx)
     {
        BigInteger result = null;
        ProjectCalendar cal = mpx.getCalendar();
        if (cal != null)
        {
           result = NumberHelper.getBigInteger(cal.getUniqueID());
        }
        else
        {
           result = NULL_CALENDAR_ID;
        }
        return (result);
     }
docs: This method retrieves the UID for a calendar associated with a task.
  
  @param mpx MPX Task instance
  @return calendar UID