JSFiddle - React, Tailwind, and code Playground

HTML

<div class="form-group">
    <label class="control-label col-sm-2" for="date">Date:</label>
    <div class="col-sm-5">
        <input id="date1" value="Pick Date" type="button" class="form-control btn btn-default" name="training_start_date" value=""> 
    </div>
</div>
<br>
<span id="pickedDate">  </span>  <span id='diff'>  </span>

JavaScript

$('#date1').datepicker({ dateFormat: 'DD, d MM, yy' });

$('#date1').change(function() {
    var date = $(this).datepicker('getDate');
    var today = new Date();
    var dayDiff = Math.ceil((today - date) / (1000*60*60*24) * -1);
    $('#date1').val($.datepicker.formatDate('DD, d MM, yy', date) + ' ( ' + dayDiff + ' Days )');
});