--- /dev/null
+<?php
+
+namespace Drupal\metatag_open_graph\Plugin\metatag\Tag;
+
+use \Drupal\metatag\Plugin\metatag\Tag\MetaPropertyBase;
+
+/**
+ * Provides a plugin for the 'book:release_date' meta tag.
+ *
+ * @MetatagTag(
+ * id = "book_release_date",
+ * label = @Translation("Release Date"),
+ * description = @Translation("The date the book was released."),
+ * name = "book:release_date",
+ * group = "open_graph",
+ * weight = 37,
+ * type = "date",
+ * secure = FALSE,
+ * multiple = FALSE
+ * )
+ */
+class BookReleaseDate extends MetaPropertyBase {
+ // Nothing here yet. Just a placeholder class for a plugin.
+}