Fix incorrectly calculating the datetime property for <time>

Closes #3394682

src/Plugin/Field/FieldFormatter/DateTimeTimezoneDefaultFormatter.php

@@ -29,7 +29,7 @@ class DateTimeTimezoneDefaultFormatter extends DateTimeTimezoneFormatterBase {
       if ($item->value) {
         // Create the ISO date in Universal Time.
         $date = $item->date;
-        $iso_date = $date->format('Y-m-d\TH:i:s', ['timezone' => $item->timezone]) . 'Z';
+        $iso_date = $date->format(\DateTimeInterface::ATOM, ['timezone' => 'UTC']);
         $build[$delta] = [
           '#theme' => 'time',
           '#text' => $this->formatDate($date, $item->timezone),