diff --git a/src/AirQuality.php b/src/AirQuality.php
index 652dd38..e8c7b29 100644
--- a/src/AirQuality.php
+++ b/src/AirQuality.php
@@ -67,7 +67,7 @@ public function getAQI(): array;
     /**
      * Returns the date/time the last measurement was taken.
      *
-     * @return \DateTime the date/time the last measurement was taken
+     * @return \DateTimeImmutable the date/time the last measurement was taken
      *
      * @throws \Exception
      */
diff --git a/src/WAQI.php b/src/WAQI.php
index 36eef16..dcbeb34 100644
--- a/src/WAQI.php
+++ b/src/WAQI.php
@@ -161,7 +161,7 @@ public function getAQI(): array
     /**
      * Returns the date/time the last measurement was taken.
      *
-     * @return \DateTime the date/time the last measurement was taken
+     * @return \DateTimeImmutable the date/time the last measurement was taken
      *
      * @throws \Exception
      */