fix: correctly apply due dates for contract and invoice upload reminders

src/Service/Teamer/AvailabilityProcessor.php

@@ -3,7 +3,7 @@
 namespace App\Service\Teamer;
 
 use App\Entity\Availability;
-use Carbon\Carbon;
+use Carbon\CarbonImmutable;
 
 class AvailabilityProcessor
 {
@@ -28,7 +28,7 @@ class AvailabilityProcessor
             /** @var Availability $record */
             $dateFrom = $record->getDateFrom();
             $year = $dateFrom->format('Y');
-            $month = (new Carbon($dateFrom))->locale('de_DE')->monthName;
+            $month = (new CarbonImmutable($dateFrom))->locale('de_DE')->monthName;
 
             if (false === isset($availabilities[$year])) {
                 $availabilities[$year] = [];