CIS 119 How long until Christmas?

Source code

function countdown() {
    var msg = null;
    var now = new Date();
    var Christmas = new Date(now.getFullYear(), 11, 25);
    if (now.getMonth() == Christmas.getMonth()) {
        if (now.getDate() == Christmas.getDate()) {
            msg = 'Merry Christmas';
        } else if (now.getDate() > Christmas.getDate()) {
            Christmas.setFullYear() = now.getFullYear() + 1;
        }
    }
    if (msg == null) {
        msg = Math.floor((Christmas.getTime() - now.getTime()) / 1000);
    }
    document.getElementById('display').innerHTML = msg;
    setTimeout(countdown, 1000);
}

window.onload = countdown;

Example output

Seconds left until Christmas: