class DateTimeFormatterBase

abstract class DateTimeFormatterBase extends FormatterBase implements ContainerFactoryPluginInterface {
    
    /**
     * 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 DateTimeDefaultFormatter.
     *
     * @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 $date_format_storage
     *   The date format entity storage.
     */
    public function __construct($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 [
            'timezone_override' => '',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $form['timezone_override'] = [
            '#type' => 'select',
            '#title' => $this->t('Time zone override'),
            '#description' => $this->t('The time zone selected here will always be used'),
            '#options' => system_time_zones(TRUE, TRUE),
            '#default_value' => $this->getSetting('timezone_override'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        if ($override = $this->getSetting('timezone_override')) {
            $summary[] = $this->t('Time zone: @timezone', [
                '@timezone' => $override,
            ]);
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            if ($item->date) {
                
                /** @var \Drupal\Core\Datetime\DrupalDateTime $date */
                $date = $item->date;
                $elements[$delta] = $this->buildDateWithIsoAttribute($date);
                if (!empty($item->_attributes)) {
                    $elements[$delta]['#attributes'] += $item->_attributes;
                    // Unset field item attributes since they have been included in the
                    // formatter output and should not be rendered in the field template.
                    unset($item->_attributes);
                }
            }
        }
        return $elements;
    }
    
    /**
     * Creates a formatted date value as a string.
     *
     * @param object $date
     *   A date object.
     *
     * @return string
     *   A formatted date string using the chosen format.
     */
    protected abstract function formatDate($date);
    
    /**
     * Sets the proper time zone on a DrupalDateTime object for the current user.
     *
     * A DrupalDateTime object loaded from the database will have the UTC time
     * zone applied to it.  This method will apply the time zone for the current
     * user, based on system and user settings.
     *
     * @param \Drupal\Core\Datetime\DrupalDateTime $date
     *   A DrupalDateTime object.
     */
    protected function setTimeZone(DrupalDateTime $date) {
        if ($this->getFieldSetting('datetime_type') === DateTimeItem::DATETIME_TYPE_DATE) {
            // A date without time has no timezone conversion.
            $timezone = DateTimeItemInterface::STORAGE_TIMEZONE;
        }
        else {
            $timezone = date_default_timezone_get();
        }
        $date->setTimeZone(timezone_open($timezone));
    }
    
    /**
     * Gets a settings array suitable for DrupalDateTime::format().
     *
     * @return array
     *   The settings array that can be passed to DrupalDateTime::format().
     */
    protected function getFormatSettings() {
        $settings = [];
        if ($this->getSetting('timezone_override') != '') {
            $settings['timezone'] = $this->getSetting('timezone_override');
        }
        return $settings;
    }
    
    /**
     * Creates a render array from a date object.
     *
     * @param \Drupal\Core\Datetime\DrupalDateTime $date
     *   A date object.
     *
     * @return array
     *   A render array.
     */
    protected function buildDate(DrupalDateTime $date) {
        $this->setTimeZone($date);
        $build = [
            '#markup' => $this->formatDate($date),
            '#cache' => [
                'contexts' => [
                    'timezone',
                ],
            ],
        ];
        return $build;
    }
    
    /**
     * Creates a render array from a date object with ISO date attribute.
     *
     * @param \Drupal\Core\Datetime\DrupalDateTime $date
     *   A date object.
     *
     * @return array
     *   A render array.
     */
    protected function buildDateWithIsoAttribute(DrupalDateTime $date) {
        // Create the ISO date in Universal Time.
        $iso_date = $date->format("Y-m-d\\TH:i:s") . 'Z';
        $this->setTimeZone($date);
        $build = [
            '#theme' => 'time',
            '#text' => $this->formatDate($date),
            '#attributes' => [
                'datetime' => $iso_date,
            ],
            '#cache' => [
                'contexts' => [
                    'timezone',
                ],
            ],
        ];
        return $build;
    }

}