diff --git a/date/date_c.php b/date/date_c.php
index ba86fb9d2..dd39e20d0 100644
--- a/date/date_c.php
+++ b/date/date_c.php
@@ -113,6 +113,7 @@ public function diff(
      * Returns the formatted date string on success or <b>FALSE</b> on failure.
      * Since PHP8, it always returns <b>STRING</b>.
      */
+    #[Pure]
     #[TentativeType]
     public function format(#[LanguageLevelTypeAware(['8.0' => 'string'], default: '')] $format): string;
 
@@ -430,6 +431,7 @@ public function diff(
      * @return string
      * Returns the formatted date string on success or <b>FALSE</b> on failure.
      */
+    #[Pure]
     #[TentativeType]
     public function format(#[LanguageLevelTypeAware(['8.0' => 'string'], default: '')] $format): string {}
 
@@ -654,6 +656,7 @@ public function __wakeup(): void {}
      * @return string
      * @link https://php.net/manual/en/datetime.format.php
      */
+    #[Pure]
     #[TentativeType]
     public function format(#[LanguageLevelTypeAware(['8.0' => 'string'], default: '')] $format): string {}