Use a Drupal filter.

templates/block/block--quisuisje.html.twig

@@ -32,7 +32,7 @@
     <li>Display name: {{ user.getDisplayName }}</li>
     <li>Email: {{ user.getEmail }}</li>
     <li>Language: {{ user.getPreferredLangcode }}</li>
-    <li>Last access: {{ user.getLastAccessedTime }}</li>
+    <li>Last access: {{ user.getLastAccessedTime|format_date }}</li>
     <li>Roles:
       <ul>
         {% for role in user.getRoles(true) %}