Performance.now() polyfill

perfnow.js

(function(w){
	var perfNow;
	var perfNowNames = ['now', 'webkitNow', 'msNow', 'mozNow'];
	if(!!w['performance']) for(var i = 0; i < perfNowNames.length; ++i)
	{
		var n = perfNowNames[i];
		if(!!w['performance'][n])
		{
			perfNow = function(){return w['performance'][n]()};
			break;
		}
	}
	if(!perfNow)
	{
		perfNow = Date.now;
	}
	w.perfNow = perfNow;
})(window);

// Usage
console.log(window.perfNow());

Note that Date.now() doesn't exist in IE8, so you may also need to add something along the lines of:

if(Date.now) {
    perfNow = Date.now();
} else {
    perfNow = +(new Date());
}

+(new Date()) // this is a number
function now() { return +(new Date()); } // this is a function that returns a number