anchor.diff:  
   function TDateTimeEx.toXml: String;
   begin
  +  if null then
  +    exit('');
  +
     case FPrecision of
       dtpYear:  result := sv(Year, 4);
       dtpMonth: result := sv(Year, 4) + '-' + sv(Month, 2);
  
positive.code: 
  function TDateTimeEx.ToString: String;
  begin
    Result := DateTimeToStr(DateTime);
  end;
  
  
negative.code: 
  function TDateTimeEx.toHL7: String;
  begin
    if null then
      exit('');
    case FPrecision of
      dtpYear:  result := sv(Year, 4);
      dtpMonth: result := sv(Year, 4) + sv(Month, 2);
  