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;
    }

}