JSFiddle - React, Tailwind, and code Playground

HTML

<input name="dtpEffectiveFrom" type="text" id="dtpEffectiveFrom" value="2014-01-01">
<input name="dtpEffectiveTo" type="text" id="dtpEffectiveTo" value="2014-02-01">

JavaScript

var effectiveFrom = new Date($("#dtpEffectiveFrom").val());
var effectiveTo = new Date($("#dtpEffectiveTo").val());

function daydiff(first, second) {
    return (second - first) / (1000 * 60 * 60 * 24);
}

var msg = "Are you applying for a " + daydiff(effectiveFrom, effectiveTo) + " day(s) leave?";
alert(msg);