Function to calculate page execution time using js_time and date

I saw a good method from another website that uses js to calculate page execution time, which is better than using asp

<script> <BR> <!-- <BR> var startTime,endTime; <BR> var d=new Date(); <BR> startTime=d.getTime(); <BR> //--> <BR> </script><script>d=new Date();endTime=d.getTime <BR>();document.write((endTime-startTime)/1000);</script>