DateTimeItemInterface

class \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem implements \Drupal\datetime\Plugin\Field\FieldType\DateTimeItemInterface extends \Drupal\Core\Field\FieldItemBase
class \Drupal\datetime_range\Plugin\Field\FieldType\DateRangeItem extends \Drupal\datetime\Plugin\Field\FieldType\DateTimeItem

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