setInterval() and clearInterval()

function check(){
 var now,sec;
 now = new Date;
 sec = now.getSeconds();
 alert(sec+' '+intervalID);
 
  if(sec == 20 || sec == 21 || sec == 22 || sec == 23)
  {
     alert("stop");
     clearInterval(intervalID);
  }
}

var intervalID= window.setInterval("check()",3000,"javascript");