Mobx + Moment.js not working

HTML

<div id="react-log"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.16.0/moment.js"></script>
<script src="https://cdn.jsdelivr.net/g/[email protected](react.min.js+react-dom.min.js),[email protected]"></script>
<script src="https://unpkg.com/[email protected]/lib/mobx.umd.min.js"></script>
<script src="https://unpkg.com/[email protected]/index.js"></script>
<script src="https://rawgit.com/mattruby/mobx-examples/master/logging-utils.js"></script>
<script>
	wrapConsole();
</script>

JavaScript

var person = {
	birthday: mobx.observable(moment())
};

mobx.autorun(function() {
	console.log('autorun: ' + person.birthday.toString());
});

person.birthday = moment(person.birthday); // Ugly but works
person.birthday.add(1, 'day'); // Does not work
person.birthday = moment();

console.log('no autorun: ' + person.birthday.toString());