Timer with stop ans start button in JavaScripts

HTML Code : 

    <input type="button" name="btn" id='btn' value="Start" onclick="to_start()";>
    <br><br>
   <div id=n1 style="z-index: 2; position: relative; right: 0px; top: 10px; background-color: #00cc33;
    width: 100px; padding: 10px; color: white; font-size:20px; border: #0000cc 2px dashed; "> </div>     <br>
    <input type="button" name="btn" id='btn' value="Start" onclick="stop()";>

JavaScripts Code : 

    <script language=javascript>
   var h=0;
   var m=0;
   var s=0;
   function to_start(){
   tm=window.setInterval('disp()',1000);
   }

   function stop(){
   window.clearInterval(tm); // stop the timer
   h=0;
   m=0;
   s =0;
   }


   function disp(){
   // Format the output by adding 0 if it is single digit //
   if(s<10){var s1='0' + s;}
   else{var s1=s;}
   if(m<10){var m1='0' + m;}
   else{var m1=m;}
   if(h<10){var h1='0' + h;}
   else{var h1=h;}
   // Display the output //
   str= h1 + ':' + m1 +':' + s1 ;
   var time=document.getElementById('n1').innerHTML=str;
   // Calculate the stop watch //
   if(s<59){
   s=s+1;
   }else{
   s=0;
   m=m+1;
   if(m==60){
   m=0;
   h=h+1;
   } // end if  m ==60
   }// end if else s < 59
   // end of calculation for next display

   }
   </script>