feed_content_type

			// 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 );
	}

	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 );
	}

	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|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().