fix: publication date format that also works for documents (#203)

* fix: publication date format that also works for documents

* style: formatting

---------

Co-authored-by: Thessa Kockelkorn <thessa@notfound.nl>
Co-authored-by: Rene <rene@64k.nl>

src/Services/Indexer/SearchItem.php

@@ -185,7 +185,8 @@ private function toDateString(?DateTime $date): ?string
         if ($date === null) {
             return null;
         }
+        $date->setTimezone(new \DateTimeZone('UTC'));
 
-        return $date->format(DateTime::ATOM);
+        return $date->format('Y-m-d\TH:i:s.u\Z');
     }
 }