我初学啊，做了个倒计时程序，但它就是不倒计时……

请会的大哥帮忙下啊
var
  Form1: TForm1;
implementation
var
  wh,wm,ws,wms: Word;
  nowtime: TDateTime;

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);
  var
  endtime: TDateTime;
begin
  nowtime := now ;
  wh := StrToInt(edt1.Text);
  wm := StrToInt(edt2.Text);
  ws := StrToInt(edt3.Text);
  wms:= 0;
  endtime := nowtime + EncodeTime(wh,wm,ws,wms);
if now >= endtime then
begin
  ShowMessage('时间到');
  Exit;
  end
  else
  begin
  DecodeTime(endtime-now,wh,wm,ws,wms);
  lbl3.Caption := IntToStr(wh)+':'+inttostr(wm)+':'+IntToStr(ws);
  end;
end;

procedure TForm1.tmr1Timer(Sender: TObject);
var
  lt : TSYSTEMTIME;
begin
  GetLocalTime(lt);
  lbl1.Caption:=IntToStr(lt.wHour)+':'+inttostr(lt.wMinute)+':'+inttostr(lt.wSecond);
  end;
end.