diff --git a/src/Api/MergeRequests.php b/src/Api/MergeRequests.php
index ed3779f6..2bb8584a 100644
--- a/src/Api/MergeRequests.php
+++ b/src/Api/MergeRequests.php
@@ -74,7 +74,9 @@ public function all($project_id = null, array $parameters = [])
     {
         $resolver = $this->createOptionsResolver();
         $datetimeNormalizer = function (Options $resolver, \DateTimeInterface $value): string {
-            return $value->format('c');
+            $utc = (new \DateTimeImmutable($value->format(\DateTimeImmutable::RFC3339_EXTENDED)))->setTimezone(new \DateTimeZone('UTC'));
+
+            return $utc->format('Y-m-d\TH:i:s.v\Z');
         };
         $resolver->setDefined('iids')
             ->setAllowedTypes('iids', 'array')
diff --git a/tests/Api/MergeRequestsTest.php b/tests/Api/MergeRequestsTest.php
index ae8225bc..227b89a0 100644
--- a/tests/Api/MergeRequestsTest.php
+++ b/tests/Api/MergeRequestsTest.php
@@ -107,11 +107,11 @@ public function shouldGetAllWithDateTimeParams(): void
         $expectedArray = $this->getMultipleMergeRequestsData();
 
         $createdAfter = new \DateTime('2018-01-01 00:00:00');
-        $createdBefore = new \DateTime('2018-01-31 00:00:00');
+        $createdBefore = new \DateTime('2018-01-31 12:00:00.123+03:00');
 
         $expectedWithArray = [
-            'created_after' => $createdAfter->format(\DATE_ATOM),
-            'created_before' => $createdBefore->format(\DATE_ATOM),
+            'created_after' => '2018-01-01T00:00:00.000Z',
+            'created_before' => '2018-01-31T09:00:00.123Z',
         ];
 
         $api = $this->getApiMock();