JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//ajax.googleapis.com/ajax/libs/jqueryui/1.11.2/jquery-ui.min.js"></script>

JavaScript

var selectedDateEnd = new Date(2015, 02, 12); // Selected from date picker
var controller = new Date(2015, 02, 12); // Las added item

// New item can only be adde 14 days after last added item
controller.getTime(+controller + 12096e5);

if(selectedDateEnd >= (controller)){
    console.log('selectedDateEnd >= controller');
    console.log(selectedDateEnd + ' >= ' + controller);
} else {
    console.log('selectedDateEnd < controller');
    console.log(selectedDateEnd + ' < ' + controller);
}