animacion controlada mediante un temporizador

gistfile1.js

// http://dailyjs.com/2010/06/03/framework-part-15/

function animate() {
  var box = document.getElementById('box'),
      duration = 1000,
      start = (new Date()).valueOf(),
      finish = start + duration,
      interval;

  interval = setInterval(function() {
    var time = (new Date()).valueOf(), frame = time > finish ? 1 : (time - start) / duration;

    // The thing being animated
    box.style.left = frame * 100 + 'px';

    if (time > finish) {
      clearInterval(interval);
    }
  }, 10);
}