class DateTimeItem

class DateTimeItem extends FieldItemBase implements DateTimeItemInterface {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultStorageSettings() {
    return [
      'datetime_type' => 'datetime',
    ] + parent::defaultStorageSettings();
  }
  
  /**
   * Value for the 'datetime_type' setting: store only a date.
   */
  const DATETIME_TYPE_DATE = 'date';
  
  /**
   * Value for the 'datetime_type' setting: store a date and time.
   */
  const DATETIME_TYPE_DATETIME = 'datetime';
  
  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['value'] = DataDefinition::create('datetime_iso8601')->setLabel(new TranslatableMarkup('Date value'))
      ->setRequired(TRUE);
    $properties['date'] = DataDefinition::create('any')->setLabel(new TranslatableMarkup('Computed date'))
      ->setDescription(new TranslatableMarkup('The computed DateTime object.'))
      ->setComputed(TRUE)
      ->setClass('\\Drupal\\datetime\\DateTimeComputed')
      ->setSetting('date source', 'value');
    return $properties;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {
    return [
      'columns' => [
        'value' => [
          'description' => 'The date value.',
          'type' => 'varchar',
          'length' => 20,
        ],
      ],
      'indexes' => [
        'value' => [
          'value',
        ],
      ],
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
    $element = [];
    $element['datetime_type'] = [
      '#type' => 'select',
      '#title' => $this->t('Date type'),
      '#description' => $this->t('Choose the type of date to create.'),
      '#default_value' => $this->getSetting('datetime_type'),
      '#options' => [
        static::DATETIME_TYPE_DATETIME => $this->t('Date and time'),
        static::DATETIME_TYPE_DATE => $this->t('Date only'),
      ],
      '#disabled' => $has_data,
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
    $type = $field_definition->getSetting('datetime_type');
    // Just pick a date in the past year. No guidance is provided by this Field
    // type.
    $timestamp = REQUEST_TIME - mt_rand(0, 86400 * 365);
    if ($type == DateTimeItem::DATETIME_TYPE_DATE) {
      $values['value'] = gmdate(static::DATE_STORAGE_FORMAT, $timestamp);
    }
    else {
      $values['value'] = gmdate(static::DATETIME_STORAGE_FORMAT, $timestamp);
    }
    return $values;
  }
  
  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $value = $this->get('value')
      ->getValue();
    return $value === NULL || $value === '';
  }
  
  /**
   * {@inheritdoc}
   */
  public function onChange($property_name, $notify = TRUE) {
    // Enforce that the computed date is recalculated.
    if ($property_name == 'value') {
      $this->date = NULL;
    }
    parent::onChange($property_name, $notify);
  }

}