Ticket #58986: ticket-patch-58986.diff

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -1802 +1802 @@
                          * with the site's timezone offset applied.
                          */
                         if ( '0000-00-00 00:00:00' === $post->post_modified_gmt ) {
-                                $post_modified_gmt = gmdate( 'Y-m-d H:i:s', strtotime( $post->post_modified ) - ( get_option( 'gmt_offset' ) * HOUR_IN_SECONDS ) );
+                                $post_modified_gmt = gmdate( 'Y-m-d H:i:s', strtotime( $post->post_modified ) - ( (int) get_option( 'gmt_offset', 0 ) * HOUR_IN_SECONDS ) );
                         } else {
                                 $post_modified_gmt = $post->post_modified_gmt;
                         }