JS performance timer

perfnow.js

var perfnow = (function() {
  return (
      (performance && performance.now)       ? function() { return performance.now();       }
    : (performance && performance.webkitNow) ? function() { return performance.webkitNow(); }
    : (performance && performance.mozNow)    ? function() { return performance.mozNow();    }
    : (performance && performance.oNow)      ? function() { return performance.oNow();      }  
    : (Date.now)                             ? function() { return Date.now();              }
    : function() { return +new Date(); }
  )
}());

http://updates.html5rocks.com/2012/08/When-milliseconds-are-not-enough-performance-now