date and time BUGFIX support for unknown timezone

Fixes #1734
diff --git a/src/tree_data.h b/src/tree_data.h
index 71db8dd..fa52499 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -671,6 +671,7 @@
 struct lyd_value_date_and_time {
     time_t time;        /**< UNIX timestamp */
     char *fractions_s;  /**< Optional fractions of a second */
+    ly_bool unknown_tz; /**< Whether the value is in the special -00:00 timezone. */
 };
 
 /**