TTestThread = class(TThread)
    private
      FTimer : TTimer;
    public
      constructor Create;
      procedure Execute; override;
      procedure TestTimer(Sender : TObject);
    end;

.
.
.


{ TTestThread }

constructor TTestThread.Create;
begin
inherited Create(False);
FreeOnTerminate:=True;
FTimer:=TTimer.Create(nil);
FTimer.Interval:=1000;
FTimer.OnTimer:=TestTimer;
FTimer.Enabled:=True;
end;

procedure TTestThread.Execute;
begin
while not Terminated do
  sleep(10);
end;

procedure TTestThread.TestTimer(Sender: TObject);
const f = 'C:\Testsl.txt';
begin
with TstringList.Create do
  try
    if Fileexists(f)then
      LoadFromFile(f);
    Add(DateTimeToStr(Now));
    SaveToFile(f);
  finally
    Free;
    end;
end;