@echo off

:zacatek

  set myvar=

  for /f "delims=" %%a in ('ping -n 1 www.xxx.yyy.zzz ^| find "TTL="') do set myvar=%%a

  if "%myvar%" == "" goto chyba

  echo %myvar%

  echo %date% %time%   %myvar% >> tstpng.txt

  goto cekej

:chyba

  echo Chyba!

  echo %date% %time%   NE >> tstpng.txt

:cekej

  timeout 15

  goto zacatek