...
type
 TMyThread = class(TThread)
 private
  {private declarations}
  timer: TTimer;
  FStartTime: TDateTime;
  procedure TimerOnTimer(Sender: TObject);
 protected
   procedure Execute; override;

end;
...

procedure TMyThread.TimerOnTimer(Sender: TObject);
begin
Form1.Edit1.Text := FormatDateTime('NN:SS', Now - FStartTime);
end;

procedure TMyThread.Execute;
begin
FStartTime := Now;
timer := TTimer.Create(nil);
timer.OnTimer := TimerOnTimer;
timer.Interval := 100;

...

while not terminated do
begin
  //Hier macht der Thread sein Hauptjob
end;
...
timer.Free;
end;