--- ---

If you need to react when system date time has changed you can handle the WM_TimeChange Windows message.

    type
       TForm1 = class(TForm)
       private
         procedure WMTimeChange(var Msg: TMessage) ; message WM_TIMECHANGE;
       end;

    var
       Form1: TForm1;

    implementation
    {$R *.DFM}

    procedure TForm1.WMTimeChange(var Msg: TMessage) ;
    begin
       inherited;
       ShowMessage('Date/Time has changed!') ;
    end;