diff --git a/src/Controller/WopiController.php b/src/Controller/WopiController.php
index 826526ec..c7d8c227 100644
--- a/src/Controller/WopiController.php
+++ b/src/Controller/WopiController.php
@@ -15,6 +15,7 @@
 use Drupal\collabora_online\Exception\CollaboraNotAvailableException;
 use Drupal\collabora_online\Jwt\JwtTranscoderInterface;
 use Drupal\collabora_online\MediaHelperInterface;
+use Drupal\collabora_online\Util\IsoDateTime;
 use Drupal\Component\Datetime\TimeInterface;
 use Drupal\Core\DependencyInjection\AutowireTrait;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
@@ -73,8 +74,6 @@ public function __construct(
    *   The response with file contents.
    */
   public function wopiCheckFileInfo(MediaInterface $media, FileInterface $file, UserInterface $user, bool $can_write): Response {
-    $mtime = date_create_immutable_from_format('U', $file->getChangedTime());
-
     if ($can_write && !$media->access('edit in collabora', $user)) {
       $this->logger->error('Token and user permissions do not match.');
       throw new AccessDeniedHttpException('The user does not have collabora edit access for this media.');
@@ -83,7 +82,7 @@ public function wopiCheckFileInfo(MediaInterface $media, FileInterface $file, Us
     $response_data = [
       'BaseFileName' => $file->getFilename(),
       'Size' => $file->getSize(),
-      'LastModifiedTime' => $mtime->format('c'),
+      'LastModifiedTime' => IsoDateTime::format($file->getChangedTime()),
       'UserId' => $user->id(),
       'UserFriendlyName' => $user->getDisplayName(),
       'UserCanWrite' => $can_write,
@@ -149,7 +148,6 @@ public function wopiPutFile(MediaInterface $media, FileInterface $file, UserInte
 
     $new_file_content = $request->getContent();
     $new_file = $this->createNewFileEntity($file, $new_file_content);
-    $mtime = date_create_immutable_from_format('U', $new_file->getChangedTime());
 
     $save_reason = $this->buildSaveReason($request);
 
@@ -163,7 +161,7 @@ public function wopiPutFile(MediaInterface $media, FileInterface $file, UserInte
 
     return new JsonResponse(
       [
-        'LastModifiedTime' => $mtime->format('c'),
+        'LastModifiedTime' => IsoDateTime::format($new_file->getChangedTime()),
       ],
       Response::HTTP_OK,
       ['content-type' => 'application/json'],
diff --git a/src/Util/IsoDateTime.php b/src/Util/IsoDateTime.php
new file mode 100644
index 00000000..bd2a5606
--- /dev/null
+++ b/src/Util/IsoDateTime.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\collabora_online\Util;
+
+/**
+ * Static methods related to date and time.
+ */
+class IsoDateTime {
+
+  /**
+   * Formats a timestamp as a date in UTC timezone in ISO 8601 format.
+   *
+   * @param int $timestamp
+   *   Unix timestamp.
+   *
+   * @return string
+   *   Formatted date string in ISO 8601 format, in UTC.
+   */
+  public static function format(int $timestamp): string {
+    // If the input is in timestamp format, the timezone is always UTC.
+    return \DateTimeImmutable::createFromFormat('U', (string) $timestamp)
+      ->format('c');
+  }
+
+}