Instantiate timestamp as DateTime

src/Cas/Protocol/Cas20.php

@@ -150,7 +150,7 @@ public function getValidateSuccessResponse(string $username): AuthenticationSucc
  }
 
  $attributes = new Attributes(
- new AuthenticationDate(gmdate('Y-m-d\TH:i:s\Z', time())),
+ new AuthenticationDate(new DateTimeImmutable('now')),
  new LongTermAuthenticationRequestTokenUsed('true'),
  new IsFromNewLogin('true'),
  $attr,