diff --git a/src/FrozenClock.php b/src/FrozenClock.php
index dfe302d3..384a1657 100644
--- a/src/FrozenClock.php
+++ b/src/FrozenClock.php
@@ -26,7 +26,7 @@ public function setTo(DateTimeImmutable $now): void
     /**
      * Adjusts the current time by a given modifier.
      *
-     * @param string $modifier @see https://www.php.net/manual/en/datetime.formats.php
+     * @param non-empty-string $modifier @see https://www.php.net/manual/en/datetime.formats.php
      *
      * @throws DateMalformedStringException When an invalid date/time string is passed.
      */
diff --git a/src/SystemClock.php b/src/SystemClock.php
index 6b65dfa5..36ccb774 100644
--- a/src/SystemClock.php
+++ b/src/SystemClock.php
@@ -9,9 +9,9 @@
 use function date_default_timezone_get;
 
 /** @immutable */
-final class SystemClock implements Clock
+final readonly class SystemClock implements Clock
 {
-    public function __construct(private readonly DateTimeZone $timezone)
+    public function __construct(private DateTimeZone $timezone)
     {
     }