diff --git a/wp-includes/feed.php b/wp-includes/feed.php
index 54f7bcdf52..c0b75016e3 100644
--- a/wp-includes/feed.php
+++ b/wp-includes/feed.php
@@ -648,55 +648,59 @@ function self_link() {
 }
 
 /**
- * Get the timestamp of the most recently modified post from WP_Query.
+ * Get the UTC time of the most recently modified post from WP_Query.
  *
- * If viewing a comment feed, the timestamp of the most recently modified
+ * If viewing a comment feed, the time of the most recently modified
  * comment will be returned.
  *
  * @global WP_Query $wp_query WordPress Query object.
  *
  * @since 5.2.0
  *
- * @param string $format Format of the timestamp to return, passed to mysql2date.
- *
- * @return string The timestamp.
+ * @param string $format Date format string to return the time in.
+ * @return string|false The time in requested format, or false on failure.
  */
 function get_feed_build_date( $format ) {
 	global $wp_query;
 
-	if ( empty( $wp_query ) || ! $wp_query->have_posts() ) {
-		// Fallback to last time any post was modified or published.
-		return get_lastpostmodified( 'GMT' );
+	$datetime          = false;
+	$max_modified_time = false;
+	$utc               = new DateTimeZone( 'UTC' );
+
+	if ( ! empty( $wp_query ) && $wp_query->have_posts() ) {
+		// Extract the post modified times from the posts.
+		$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );
+
+		// If this is a comment feed, check those objects too.
+		if ( $wp_query->is_comment_feed() && $wp_query->comment_count ) {
+			// Extract the comment modified times from the comments.
+			$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );
+
+			// Add the comment times to the post times for comparison.
+			$modified_times = array_merge( $modified_times, $comment_times );
+		}
+
+		// Determine the maximum modified time.
+		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', max( $modified_times ), $utc );
 	}
 
-	// Extract the post modified times from the posts.
-	$modified_times = wp_list_pluck( $wp_query->posts, 'post_modified_gmt' );
-
-	// If this is a comment feed, check those objects too.
-	if ( $wp_query->is_comment_feed() && $wp_query->comment_count ) {
-		// Extract the comment modified times from the comments.
-		$comment_times = wp_list_pluck( $wp_query->comments, 'comment_date_gmt' );
-
-		// Add the comment times to the post times for comparison.
-		$modified_times = array_merge( $modified_times, $comment_times );
+	if ( false === $datetime ) {
+		// Fall back to last time any post was modified or published.
+		$datetime = date_create_immutable_from_format( 'Y-m-d H:i:s', get_lastpostmodified( 'GMT' ), $utc );
 	}
 
-	// Determine the maximum modified time.
-	$datetime = date_create_immutable_from_format(
-		'Y-m-d H:i:s',
-		max( $modified_times ),
-		new DateTimeZone( 'UTC' )
-	);
-
-	$max_modified_time = $datetime->format( $format );
+	if ( false !== $datetime ) {
+		$max_modified_time = $datetime->format( $format );
+	}
 
 	/**
 	 * Filters the date the last post or comment in the query was modified.
 	 *
 	 * @since 5.2.0
 	 *
-	 * @param string $max_modified_time Date the last post or comment was modified in the query.
-	 * @param string $format            The date format requested in get_feed_build_date.
+	 * @param string|false $max_modified_time Date the last post or comment was modified in the query, in UTC.
+	 *                                        False on failure.
+	 * @param string       $format            The date format requested in get_feed_build_date().
 	 */
 	return apply_filters( 'get_feed_build_date', $max_modified_time, $format );
 }
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 2b2735bdc4..aa81525396 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.4-alpha-46973';
+$wp_version = '5.4-alpha-46974';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.