HTML5 Canvas Rotating Plane

window.requestAnimFrame = (function(callback){
    return window.requestAnimationFrame ||
    window.webkitRequestAnimationFrame ||
    window.mozRequestAnimationFrame ||
    window.oRequestAnimationFrame ||
    window.msRequestAnimationFrame ||
    function(callback){
        window.setTimeout(callback, 1000 / 60);
    };
})();
 
function animate(plane, lastTime){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
 
    // update
    var date = new Date();
    var time = date.getTime();
    var timeDiff = lastTime - time;
    plane.theta += timeDiff * plane.rotationSpeed / 1000;
    plane.tilt = Math.sin(plane.theta / 2);
 
    // clear
    context.clearRect(0, 0, canvas.width, canvas.height);
 
    // draw
    context.save();
    context.translate(canvas.width / 2, canvas.height / 2);
    context.scale(1, plane.tilt);
    context.rotate(plane.theta);
    var planeColor = plane.tilt >= 0 ? "red" : "blue";
 
    context.fillStyle = planeColor;
    context.fillRect(-plane.width / 2, -plane.height / 2, plane.width, plane.height);
    context.restore();
 
    // request new frame
    requestAnimFrame(function(){
        animate(plane, time);
    });
}
 
window.onload = function(){
    var plane = {
        width: 200,
        height: 100,
        theta: 0,
        tilt: 1,
        rotationSpeed: 2
    };
 
    var date = new Date();
    var time = date.getTime();
    animate(plane, time);
};