class DateTimeCustomFormatter

class DateTimeCustomFormatter extends DateTimeFormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'date_format' => DateTimeItemInterface::DATETIME_STORAGE_FORMAT,
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        // @todo Evaluate removing this method in
        // https://www.drupal.org/node/2793143 to determine if the behavior and
        // markup in the base class implementation can be used instead.
        $elements = [];
        foreach ($items as $delta => $item) {
            if (!empty($item->date)) {
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
                $date = $item->date;
                $elements[$delta] = $this->buildDate($date);
            }
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function formatDate($date) {
        $format = $this->getSetting('date_format');
        $timezone = $this->getSetting('timezone_override') ?: $date->getTimezone()
            ->getName();
        return $this->dateFormatter
            ->format($date->getTimestamp(), 'custom', $format, $timezone != '' ? $timezone : NULL);
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['date_format'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Date/time format'),
            '#description' => $this->t('See <a href="https://www.php.net/manual/datetime.format.php#refsect1-datetime.format-parameters" target="_blank">the documentation for PHP date formats</a>.'),
            '#default_value' => $this->getSetting('date_format'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        $date = new DrupalDateTime();
        $this->setTimeZone($date);
        $summary[] = $date->format($this->getSetting('date_format'), $this->getFormatSettings());
        return $summary;
    }

}