Functions that calculate page execution time using js

I saw a good page execution time using js I used to calculate the page execution time, which is better than using asp
<script language=javascript>
 <!--
 var startTime,endTime;
 var d=new Date();
 startTime=();
 //-->
 </script>
<script language=javascript>d=new Date();endTime=
();((endTime-startTime)/1000);</script>