now()

A cross-browser, Javascript shim function to return the number of milliseconds elapsed since either the browser navigationStart event (using performance.now or browser equivalent) or the UNIX epoch, depending on availability.

HTML

<form>
         <input type="button" value="Click Me" onclick="show()" />
      </form>

CSS

#output {font-size: 30px;}

JavaScript

var now = window.performance.now;

var start = now();

 function show(){
               var current = now();
               var duration = start - current;
               alert(duration);
            }