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', ];
This component doesn't work for logged-in users when the quick-edit module is enabled, because of conflicting JS.