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

src/Entity/Assignment.php

@@ -6,7 +6,7 @@ use App\Entity\Traits\BlameableEntity;
 use App\Entity\Traits\SoftDeletableEntity;
 use App\Entity\Traits\TimestampableEntity;
 use App\Repository\AssignmentRepository;
-use Carbon\CarbonPeriod;
+use Carbon\CarbonPeriodImmutable;
 use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
 use Doctrine\DBAL\Types\Types;
@@ -270,13 +270,19 @@ class Assignment implements BlameableEntityInterface, TimestampableEntityInterfa
         return $this;
     }
 
-    public function getEffectivePeriod(): ?CarbonPeriod
+    /**
+     * Immutable on purpose: a plain CarbonPeriod hands out mutable Carbon instances, so a caller
+     * deriving one date from another - `$end = $period->getEndDate(); $end->addDays(14)` - moves
+     * the original as well and silently collapses its own period. CarbonPeriodImmutable yields
+     * CarbonImmutable from start, end and getEndDate(), which makes that mistake impossible.
+     */
+    public function getEffectivePeriod(): ?CarbonPeriodImmutable
     {
         if (null !== $destination = $this->getDestination()) {
             $dateFrom = $this->getDateFrom() ?? $destination->getDateFrom();
             $dateTo = $this->getDateTo() ?? $destination->getDateTo();
 
-            return new CarbonPeriod($dateFrom, $dateTo);
+            return new CarbonPeriodImmutable($dateFrom, $dateTo);
         }
 
         return null;