Create timer

Use Timer component and this code:

var
  MyTime: TDateTime;
...
procedure TForm1.Button1Click(Sender: TObject);
begin
  Timer1.Interval:=1000;
  MyTime:=Now;
  Timer1Timer(Timer1);
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Label1.Caption:=TimeToStr(Now-MyTime);
end;