javascriptデジタル時計を実現します.

<div id="clock"></div>

function changeClock() {

    var d = new Date();

    var mm = (d.getMinutes() < 10) ? ('0' + d.getMinutes()):d.getMinutes();  //        10     0

    var ss = (d.getSeconds() < 10) ? ('0' + d.getSeconds()):d.getSeconds();

    document.getElementById("clock").innerHTML = d.getHours() + ":" + mm + ":" + ss;

}

window.setInterval(changeClock, 1000);