Code:
function UpTime: string;
const
  ticksperday: Integer    = 1000 * 60 * 60 * 24;
  ticksperhour: Integer   = 1000 * 60 * 60;
  ticksperminute: Integer = 1000 * 60;
  tickspersecond: Integer = 1000;
var
  t:          Longword;
  d, h, m, s: Integer;
begin
  t := GetTickCount;

  d := t div ticksperday;
  Dec(t, d * ticksperday);

  h := t div ticksperhour;
  Dec(t, h * ticksperhour);

  m := t div ticksperminute;
  Dec(t, m * ticksperminute);

  s := t div tickspersecond;

  Result := 'Uptime: ' + IntToStr(d) + ' Days ' + IntToStr(h) + ' Hours ' + IntToStr(m) +
    ' Minutes ' + IntToStr(s) + ' Seconds';
end;


procedure TForm1.Button1Click(Sender: TObject);
begin
  label1.Caption := UpTime;
end;