Ciao cambia questo pezzo:
codice:
  document.getElementById("remain").innerHTML = days+" days, "+hours+" hours, "+minutes+" minutes, "+seconds+" seconds";
  setTimeout ( "setCountDown()", 1000 );
con questo:
codice:
 if(days == 0 && hours == 0 && minutes == 0 && seconds == 0){document.getElementById("remain").innerHTML ="Offerta terminata"}else{document.getElementById("remain").innerHTML = days+" days, "+hours+" hours, "+minutes+" minutes, "+seconds+" seconds";
  setTimeout ( "setCountDown()", 1000 );}