JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://chmln.github.io/flatpickr/dist/flatpickr.min.css">
<script src="https://chmln.github.io/flatpickr/dist/flatpickr.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.6.3/css/font-awesome.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.6/css/bootstrap.css">
<h4>Without altInput (data-clear is working)</h4>

<div id="flatpickr1" class="input-group" data-wrap="true">
  <div class="input-group-addon" data-clear>
    <span class="fa fa-remove"></span>
  </div>
  <input type="text" class="form-control" data-input data-open>
  <div class="input-group-addon" data-toggle>
    <span class="fa fa-calendar"></span>
  </div>
</div>

<h4>With altInput (data-clear doesn't work) </h4>

<div id="flatpickr2" class="input-group" data-wrap="true">
  <div class="input-group-addon" data-clear>
    <span class="fa fa-remove"></span>
  </div>
  <input type="text" class="form-control" data-input data-open>
  <div class="input-group-addon" data-toggle>
    <span class="fa fa-calendar"></span>
  </div>
</div>

CSS

#flatpickr1,
#flatpickr2 {
  max-width: 30rem;
}

JavaScript

flatpickr("#flatpickr1", {
  dateFormat: 'Y-m-d',
});

flatpickr("#flatpickr2", {
  dateFormat: 'Y-m-d',
  altInput: false,
  altFormat: 'd/m/Y',
  altInputClass: "form-control"
});

console.log(calendar);