class TimestampFormatter

class TimestampFormatter extends FormatterBase {
  
  /**
   * The date formatter service.
   *
   * @var \Drupal\Core\Datetime\DateFormatterInterface
   */
  protected $dateFormatter;
  
  /**
   * The date format entity storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $dateFormatStorage;
  
  /**
   * Constructs a new TimestampFormatter.
   *
   * @param string $plugin_id
   *   The plugin_id for the formatter.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
   *   The definition of the field to which the formatter is associated.
   * @param array $settings
   *   The formatter settings.
   * @param string $label
   *   The formatter label display setting.
   * @param string $view_mode
   *   The view mode.
   * @param array $third_party_settings
   *   Third party settings.
   * @param \Drupal\Core\Datetime\DateFormatterInterface $date_formatter
   *   The date formatter service.
   * @param \Drupal\Core\Entity\EntityStorageInterface $date_format_storage
   *   The date format storage.
   */
  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, $label, $view_mode, array $third_party_settings, DateFormatterInterface $date_formatter, EntityStorageInterface $date_format_storage) {
    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $label, $view_mode, $third_party_settings);
    $this->dateFormatter = $date_formatter;
    $this->dateFormatStorage = $date_format_storage;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static($plugin_id, $plugin_definition, $configuration['field_definition'], $configuration['settings'], $configuration['label'], $configuration['view_mode'], $configuration['third_party_settings'], $container->get('date.formatter'), $container->get('entity_type.manager')
      ->getStorage('date_format'));
  }
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'date_format' => 'medium',
      'custom_date_format' => '',
      'timezone' => '',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $elements = parent::settingsForm($form, $form_state);
    $date_formats = [];
    foreach ($this->dateFormatStorage
      ->loadMultiple() as $machine_name => $value) {
      $date_formats[$machine_name] = $this->t('@name format: @date', [
        '@name' => $value->label(),
        '@date' => $this->dateFormatter
          ->format(REQUEST_TIME, $machine_name),
      ]);
    }
    $date_formats['custom'] = $this->t('Custom');
    $elements['date_format'] = [
      '#type' => 'select',
      '#title' => $this->t('Date format'),
      '#options' => $date_formats,
      '#default_value' => $this->getSetting('date_format') ?: 'medium',
    ];
    $elements['custom_date_format'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Custom date 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('custom_date_format') ?: '',
    ];
    $elements['custom_date_format']['#states']['visible'][] = [
      ':input[name="fields[' . $this->fieldDefinition
        ->getName() . '][settings_edit_form][settings][date_format]"]' => [
        'value' => 'custom',
      ],
    ];
    $elements['timezone'] = [
      '#type' => 'select',
      '#title' => $this->t('Time zone'),
      '#options' => [
        '' => $this->t('- Default site/user time zone -'),
      ] + system_time_zones(FALSE, TRUE),
      '#default_value' => $this->getSetting('timezone'),
    ];
    return $elements;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = parent::settingsSummary();
    $date_format = $this->getSetting('date_format');
    $summary[] = $this->t('Date format: @date_format', [
      '@date_format' => $date_format,
    ]);
    if ($this->getSetting('date_format') === 'custom' && $custom_date_format = $this->getSetting('custom_date_format')) {
      $summary[] = $this->t('Custom date format: @custom_date_format', [
        '@custom_date_format' => $custom_date_format,
      ]);
    }
    if ($timezone = $this->getSetting('timezone')) {
      $summary[] = $this->t('Time zone: @timezone', [
        '@timezone' => $timezone,
      ]);
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    $date_format = $this->getSetting('date_format');
    $custom_date_format = '';
    $timezone = $this->getSetting('timezone') ?: NULL;
    $langcode = NULL;
    // If an RFC2822 date format is requested, then the month and day have to
    // be in English. @see http://www.faqs.org/rfcs/rfc2822.html
    if ($date_format === 'custom' && ($custom_date_format = $this->getSetting('custom_date_format')) === 'r') {
      $langcode = 'en';
    }
    foreach ($items as $delta => $item) {
      $elements[$delta] = [
        '#cache' => [
          'contexts' => [
            'timezone',
          ],
        ],
        '#markup' => $this->dateFormatter
          ->format($item->value, $date_format, $custom_date_format, $timezone, $langcode),
      ];
    }
    return $elements;
  }

}