performance.now() polyfill (aka perf.now())

performance.now()-polyfill.js

// @license http://opensource.org/licenses/MIT
// copyright Paul Irish 2015


// Date.now() is supported everywhere except IE8. For IE8 we use the Date.now polyfill
//   github.com/Financial-Times/polyfill-service/blob/master/polyfills/Date.now/polyfill.js
// as Safari 6 doesn't have support for NavigationTiming, we use a Date.now() timestamp for relative values

// if you want values similar to what you'd get with real perf.now, place this towards the head of the page
// but in reality, you're just getting the delta between now() calls, so it's not terribly important where it's placed


(function(){

  if ("performance" in window == false) {
      window.performance = {};
  }
  
  Date.now = (Date.now || function () {  // thanks IE8
	  return new Date().getTime();
  });

  if ("now" in window.performance == false){
    
    var nowOffset = Date.now();
    
    if (performance.timing && performance.timing.navigationStart){
      nowOffset = performance.timing.navigationStart
    }

    window.performance.now = function now(){
      return Date.now() - nowOffset;
    }
    if (!performance.timing) {
    	performance.timing = {};
    }
    if (!performance.timing.navigationStart) {
    	performance.timing.navigationStart =  function() {
    		return nowOffset;
    	}
    }
  }

})();

Added polyfill for performance.timing.navigationStart also.

Needed a workaround to use a thirdparty lib that checked if performance.now existed and then used performance.timing.navigationStart.