class Date

class Date extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#theme' => 'input__date',
            '#process' => [
                [
                    $class,
                    'processAjaxForm',
                ],
                [
                    $class,
                    'processDate',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderDate',
                ],
            ],
            '#theme_wrappers' => [
                'form_element',
            ],
            '#attributes' => [
                'type' => 'date',
            ],
            '#date_date_format' => 'Y-m-d',
        ];
    }
    
    /**
     * Processes a date form element.
     *
     * @param array $element
     *   The form element to process. Properties used:
     *   - #attributes: An associative array containing:
     *     - type: The type of date field rendered.
     *   - #date_date_format: The date format used in PHP formats.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $complete_form
     *   The complete form structure.
     *
     * @return array
     *   The processed element.
     */
    public static function processDate(&$element, FormStateInterface $form_state, &$complete_form) {
        // Attach JS support for the date field, if we can determine which date
        // format should be used.
        if ($element['#attributes']['type'] == 'date' && !empty($element['#date_date_format'])) {
            $element['#attached']['library'][] = 'core/drupal.date';
            $element['#attributes']['data-drupal-date-format'] = [
                $element['#date_date_format'],
            ];
        }
        return $element;
    }
    
    /**
     * Adds form-specific attributes to a 'date' #type element.
     *
     * Supports HTML5 types of 'date', 'datetime', 'datetime-local', and 'time'.
     * Falls back to a plain textfield with JS datepicker support. Used as a
     * sub-element by the datetime element type.
     *
     * @param array $element
     *   An associative array containing the properties of the element.
     *   Properties used: #title, #value, #options, #description, #required,
     *   #attributes, #id, #name, #type, #min, #max, #step, #value, #size. The
     *   #name property will be sanitized before output. This is currently done by
     *   initializing Drupal\Core\Template\Attribute with all the attributes.
     *
     * @return array
     *   The $element with prepared variables ready for #theme 'input__date'.
     */
    public static function preRenderDate($element) {
        if (empty($element['#attributes']['type'])) {
            $element['#attributes']['type'] = 'date';
        }
        Element::setAttributes($element, [
            'id',
            'name',
            'type',
            'min',
            'max',
            'step',
            'value',
            'size',
        ]);
        static::setAttributes($element, [
            'form-' . $element['#attributes']['type'],
        ]);
        return $element;
    }

}