JSFiddle - React, Tailwind, and code Playground

by A B

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.8/themes/base/jquery-ui.css">
<input type="text" id="equipment_purchaseDate" name="equipment_purchaseDate" />
<br />
<input type="text" id="equipment_warrantyLength"  name="equipment_warrantyLength" />
<input type="text" id="equipment_warrentyExpires" name="equipment_warrentyExpires" />

JavaScript

$('#equipment_warrentyExpires').datepicker({
    dateFormat: 'dd/mm/yy'
});
$('#equipment_purchaseDate').datepicker({
    dateFormat: 'dd/mm/yy',
    onSelect: function(dateText, inst) {
        alert(dateText);
        var d = $.datepicker.parseDate('dd/mm/yy', dateText);
        alert(d);
        d.setDate(d.getDate() + ($('#equipment_warrantyLength').val() * 365) + 1);
        //multiple warranty length by 365 to get years, then add to startDate
        $('#equipment_warrentyExpires').datepicker('setDate', d);
    }
});