class DateTimeDefaultFormatter

class DateTimeDefaultFormatter extends DateTimeFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'format_type' => 'medium',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    protected function formatDate($date) {
        $format_type = $this->getSetting('format_type');
        $timezone = $this->getSetting('timezone_override') ?: $date->getTimezone()
            ->getName();
        return $this->dateFormatter
            ->format($date->getTimestamp(), $format_type, '', $timezone != '' ? $timezone : NULL);
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $time = new DrupalDateTime();
        $format_types = $this->dateFormatStorage
            ->loadMultiple();
        $options = [];
        foreach ($format_types as $type => $type_info) {
            $format = $this->dateFormatter
                ->format($time->getTimestamp(), $type);
            $options[$type] = $type_info->label() . ' (' . $format . ')';
        }
        $form['format_type'] = [
            '#type' => 'select',
            '#title' => $this->t('Date format'),
            '#description' => $this->t("Choose a format for displaying the date. Be sure to set a format appropriate for the field, i.e. omitting time for a field that only has a date."),
            '#options' => $options,
            '#default_value' => $this->getSetting('format_type'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        $date = new DrupalDateTime();
        $summary[] = $this->t('Format: @display', [
            '@display' => $this->formatDate($date),
        ]);
        return $summary;
    }

}