Mostrar la hora actual en tiempo real

function startTime() {
    today = new Date();
    h = today.getHours();
    m = today.getMinutes();
    s = today.getSeconds();
    m = checkTime(m);
    s = checkTime(s);
    document.getElementById('reloj').innerHTML = h + ":" + m + ":" + s;
    t = setTimeout('startTime()',500);
}

function checkTime(i) {
    if ( i < 10 ) {
        i = "0" + i;
    }

    return i;
}

window.onload = function(){ startTime() ;}