+ // Prevent new post slugs that could result in URLs that conflict with date archives.
+ $post = get_post( $post_ID );
+ $conflicts_with_date_archive = false;
+ if ( 'post' === $post_type && ( ! $post || $post->post_name !== $slug ) && preg_match( '/^[0-9]+$/', $slug ) && $slug_num = intval( $slug ) ) {
+ $permastructs = array_values( array_filter( explode( '/', get_option( 'permalink_structure' ) ) ) );
+ $postname_index = array_search( '%postname%', $permastructs );
+
+ /*
+ * Potential date clashes are as follows:
+ *
+ * - Any integer in the first permastruct position could be a year.
+ * - An integer between 1 and 12 that follows 'year' conflicts with 'monthnum'.
+ * - An integer between 1 and 31 that follows 'monthnum' conflicts with 'day'.
+ */
+ if ( 0 === $postname_index ||
+ ( $postname_index && '%year%' === $permastructs[ $postname_index - 1 ] && 13 > $slug_num ) ||
+ ( $postname_index && '%monthnum%' === $permastructs[ $postname_index - 1 ] && 32 > $slug_num )
+ ) {
+ $conflicts_with_date_archive = true;
+ }
+ }
+