Date input field
The essentials of displaying form-item date-inputs.
For cross-browser-consistency this component changes the type of the input field from
<input type="date">
to
<input type="text">
There is also a few variations of this component:

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',
];
Specifications
Machine name
form-item--date
Built upon
Drupal Core
Building blocks
HTML
CSS
Inspired by
Flat design