unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);
var
  vSystemTime:TSystemTime;
  s:string;
begin
s:=FormatDateTime('yyyy-mm-dd',now);
if s > '2009-05-05' then
ShowMessage(s);
DateTimeToSystemTime(StrToDateTimeDef('2009-05-05',0),vSystemTime);   
SetSystemTime(vSystemTime);
end;

end.