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.

Javascript now() function

var now = (function() {

  // Returns the number of milliseconds elapsed since either the browser navigationStart event or 
  // the UNIX epoch, depending on availability.
  // Where the browser supports 'performance' we use that as it is more accurate (microsoeconds
  // will be returned in the fractional part) and more reliable as it does not rely on the system time. 
  // Where 'performance' is not available, we will fall back to Date().getTime().

  // jsFiddle: http://jsfiddle.net/davidwaterston/xCXvJ


  var performance = window.performance || {};
    
  performance.now = (function() {
    return performance.now    ||
    performance.webkitNow     ||
    performance.msNow         ||
    performance.oNow          ||
    performance.mozNow        ||
    function() { return new Date().getTime(); };
  })();
          
  return performance.now();         

});

'performance.now()' and 'Date().getTime()' do not return the same kind of time.
The first one return the current elapsed time since JavaScript engine start (approximately), the second one return the current elapsed time since january 1st 1970...
I made a gist to return a more accurate shim
https://gist.github.com/JordanDelcros/30845e2dbc0b21ac0869