diff --git a/src/SystemClock.php b/src/SystemClock.php
index 69de81f8..6b65dfa5 100644
--- a/src/SystemClock.php
+++ b/src/SystemClock.php
@@ -8,6 +8,7 @@
 
 use function date_default_timezone_get;
 
+/** @immutable */
 final class SystemClock implements Clock
 {
     public function __construct(private readonly DateTimeZone $timezone)