TDBChart : get values from Graph OnClickSeries

I use this code to get values on event OnClickSeries,
but Date is invalid.
Before i attempted to set DBChart1.BottomAxis.DateTimeFormat='dd/mm/yy';
but it not help.
I will glad to get suggestions. What is wrong in my code
Thanks
Oleg

procedure TfrmGraph.DBChart1ClickSeries(Sender TCustomChart;
Series TChartSeries; ValueIndex Integer; Button TMouseButton;
Shift TShiftState; X, Y Integer);

begin
Sender.Hint ='';
Sender.Hint = Series.YValues.ValueSource + #13#10+
'Date and Time '+ Series.XValueToText(Series.XValue[valueindex]) +#13#10+
'Value ' + Floattostr(Series.YValue[valueindex]);
Sender.ShowHint = True;
end;