diff --git tests/phpunit/tests/comment/query.php tests/phpunit/tests/comment/query.php
index 3f56ee2..21484b2 100644
--- tests/phpunit/tests/comment/query.php
+++ tests/phpunit/tests/comment/query.php
@@ -1081,6 +1081,48 @@ class Tests_Comment_Query extends WP_UnitTestCase {
 		$this->assertNotContains( 'ORDER BY', $q->request );
 	}
 
+	/**
+	 * @ticket 30478
+	 */
+	public function test_orderby_date_modified_gmt_should_order_by_comment_ID_in_case_of_tie_ASC() {
+		$now = current_time( 'mysql', 1 );
+		$comments = $this->factory->comment->create_many( 5, array(
+			'comment_post_ID' => $this->post_id,
+			'comment_date_gmt' => $now,
+		) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'orderby' => 'comment_date_gmt',
+			'order' => 'ASC',
+		) );
+
+		// $comments is ASC by default.
+		$this->assertEquals( $comments, wp_list_pluck( $found, 'comment_ID' ) );
+	}
+
+	/**
+	 * @ticket 30478
+	 */
+	public function test_orderby_date_modified_gmt_should_order_by_comment_ID_in_case_of_tie_DESC() {
+		$now = current_time( 'mysql', 1 );
+		$comments = $this->factory->comment->create_many( 5, array(
+			'comment_post_ID' => $this->post_id,
+			'comment_date_gmt' => $now,
+		) );
+
+		$q = new WP_Comment_Query();
+		$found = $q->query( array(
+			'orderby' => 'comment_date_gmt',
+			'order' => 'DESC',
+		) );
+
+		// $comments is ASC by default.
+		rsort( $comments );
+
+		$this->assertEquals( $comments, wp_list_pluck( $found, 'comment_ID' ) );
+	}
+
 	public function test_count() {
 		$c1 = $this->factory->comment->create( array( 'comment_post_ID' => $this->post_id, 'user_id' => 7 ) );
 		$c2 = $this->factory->comment->create( array( 'comment_post_ID' => $this->post_id, 'user_id' => 7 ) );