JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/knockout/knockout-3.0.0.js"></script>
<span data-bind="text: StartDate"></span>
<br />
<span data-bind="text: EndDate"></span>

JavaScript

var ViewModel = function () {
    var self = this;

    self.StartDate = ko.observable(new Date("2014-06-24"));
    self.EndDate = ko.computed(function () {
        var myDate = new Date(self.StartDate());
        myDate.setMonth(myDate.getMonth() + 13)
        return myDate;
    });
};

$(function () {
    var vm = new ViewModel();
    ko.applyBindings(vm);
});