function now(){
        // http://updates.html5rocks.com/2012/05/requestAnimationFrame-API-now-with-sub-millisecond-precision
        return (perf && perf.now) ?
            (perf.now() + perf.timing.navigationStart) :
            Date.now();
    }
class Physics.util.ticker

The Ticker _singleton_ for easily binding callbacks to animation loops (requestAnimationFrame).

Requires window.requestAnimationFrame... so polyfill it if you need to.