this undefined in module pattern

I'm trying to use the Revealing Module pattern together with the Constructor pattern to create somewhat beautiful JS! I want to be able to create multiple instances if "Countdown" like this:

var c1 = new Countdown();
c1.init();
var c2 = new Countdown();
c2.init();

These should be independent. So instead of declaring variables with "var", I use "this" on the prototype. I reference "this" inside a function, but it's undefined. How can I access "this"?

var Countdown = function() {};

Countdown.prototype = (function(doc) {

    return {
        init: init
    };

    function init() {
        // day
        this.d1 = doc.createElement('div');
        this.d1.setAttribute('class', 'day-1 elem');

        // ... more elements left out here
    }

    function updateView(time) {
        this.d1.textContent = getDays(secs)[0];
    }

    function getDays(secs) {
        var result = 0;
        if (secs > 0) {
            result = secs / (60 * 60 * 24);
        }

        return Math.floor(result);
    }

})(document);