Apply suggestions from code review

Co-authored-by: Mikkel Ricky <[email protected]>

Author: jekuaitk

README.md

@@ -18,7 +18,7 @@ api users.
 A user can access the Webform REST API if
 
 1. it has the “OS2Form REST API user” (`os2forms_rest_api_user`) role,
-2. has been granted access to the formular
+2. has been granted access to the form
    (see [Custom access control](#custom-access-control) )
 3. has a generated key (User > Edit > Key authentication; `/user/«user
    id»/key-auth`).
@@ -131,16 +131,16 @@ Response:
 ### Webform submissions
 
 You can filter results based on submission time by
-adding query parameters to the url:
+adding query parameters to the URL:
 
 | Name        | Value                | Example      |
 |-------------|----------------------|--------------|
-| `starttime` | PHP DateTime formats | `yesterday`  |
+| `starttime` | [PHP Date and Time Formats](https://www.php.net/manual/en/datetime.formats.php) | `yesterday`  |
 | `endtime`   | PHP DateTime formats | `2023-10-23` |
 
 If left out, filtering upon the left out parameter will not be done.
 
-The example beneath requests all submissions after October 1. 2023.
+This example requests all submissions after on or after October 1st, 2023:
 
 Request:
 
@@ -151,13 +151,14 @@ Request:
 Response:
 
 ```json
-{"webform_id":"some_webform_id",
-  "starttime":"2023-10-01",
-  "submissions":{
-    "123":"https:\/\/127.0.0.1:8000\/da\/webform_rest\/some_webform_id\/submission\/44b1fe1b-ee96-481e-b941-d1219d1dcb55",
-    "124":"https:\/\/127.0.0.1:8000\/da\/webform_rest\/some_webform_id\/submission\/3652836d-3dab-4919-b880-e82cbbf3c24c"
+{
+  "webform_id": "some_webform_id",
+  "starttime": "2023-10-01",
+  "submissions": {
+    "123": "https://127.0.0.1:8000/da/webform_rest/some_webform_id/submission/44b1fe1b-ee96-481e-b941-d1219d1dcb55",
+    "124": "https://127.0.0.1:8000/da/webform_rest/some_webform_id/submission/3652836d-3dab-4919-b880-e82cbbf3c24c"
   }
-}
+}```
 ```
 
 ## Custom access control

src/Plugin/rest/resource/WebformAllFormSubmissions.php

@@ -175,14 +175,13 @@ public function get(string $webform_id): ModifiedResourceResponse {
    * @phpstan-return array<string, mixed>
    */
   private function updateSubmissionQuery(Request $request, QueryInterface $submissionQuery, string $parameter, string $operator, array &$result): array {
-    // Handle starttime request query.
-    $timeQuery = $request->query->get($parameter);
+    $value = $request->query->get($parameter);
 
     if (!empty($timeQuery)) {
       try {
-        $startTime = new \DateTime($timeQuery);
+        $startTime = new \DateTimeImmutable($timeQuery);
         $submissionQuery->condition('created', $startTime->getTimestamp(), $operator);
-        $result[$parameter] = $timeQuery;
+        $result[$parameter] = $timeQuery->format(\DateTimeImmutable::ATOM);
       }
       catch (\Exception $e) {
         $errors = [