window.onload=function(){

                        setTime();

                        setInterval('setTime()',1000);

                    }

                    function checkTime(n){

                          if(n<10){

                               return  "0" +n;

                          }else{

                              return ""+n;

                          }

                    }

                    function setTime(){

                           var date=new Date();

                           var  h=checkTime(date.getHours());

                           var  m=checkTime(date.getMinutes());

                           var  s=checkTime(date.getSeconds());

                           var time=document.getElementById('time');

                            time.innerHTML=h+":"+m+":"+s;

                    }

<body>

        <div id="time"></div>

</body>