JSFiddle - React, Tailwind, and code Playground

by Abra Cadabra

HTML

<p>firstdate:
    <input type="text" id="dp1" />
</p>
<div>
    <select name="sel" id="sel">
        <option value="1">1 week</option>
        <option value="2">2 week</option>
        <option value="3">3 week</option>
        <option value="4">4 week</option>
    </select>
</div>
<p>seconddate:
    <input type="text" id="dp2" />
</p>

JavaScript

$(function () {
    var $dp1 = $("#dp1").datepicker({
        dateFormat: 'yy-mm-dd',
        onSelect: function(){
            var currentDate = $(this).datepicker('getDate');
            $dp2.datepicker('option', 'minDate', currentDate);
        }
    });
    var $dp2 = $("#dp2").datepicker({
        dateFormat: 'yy-mm-dd'
    });

    $("#sel").on('change', function () {
        var self = $(this);
        var date2 = $dp1.datepicker('getDate');
        var daysDiff = 7 * parseInt(self.find(':selected').val());
        date2.setDate(date2.getDate() + daysDiff);
        $dp2.datepicker("setDate", date2);
    });
});