Date input field with datepicker

This is an alternate version of form-item--date that uses the jQuery UI Datepicker.
A variation of this component that also supports Select2 within the Datepicker can be found here.
Defining a date input field
In the form definition you can create a date input field as follows:
$form['start_date'] = [
'#type' => 'date',
'#title' => $this->t('Start date'),
'#date_date_format' => 'd/m/Y',
];
Warning
This component doesn't work for logged-in users when the quick-edit module is enabled, because of conflicting JS.
Specifications
Machine name
form-item--date
Built upon
Drupal Core
Drawbacks
Uses jQuery
Not accessible
Building blocks
HTML
CSS
Javascript
Inspired by
Flat design