Add better timezone support.

app/Casts/SeparateTimezoneCaster.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace FireflyIII\Casts;
+
+use Carbon\Carbon;
+use Carbon\CarbonImmutable;
+use Illuminate\Contracts\Database\Eloquent\CastsAttributes;
+use Illuminate\Database\Eloquent\Model;
+
+/**
+ * Class SeparateTimezoneCaster
+ *
+ * Checks if the object has a separate _tz value. If it does, it will use that timezone to parse the date.
+ * If it is NULL, it will use the system's timezone.
+ *
+ * At some point a user's database consists entirely of UTC dates, and we won't need this anymore. However,
+ * the completeness of this migration is not yet guaranteed.
+ */
+class SeparateTimezoneCaster implements CastsAttributes
+{
+    /**
+     * Cast the given value.
+     *
+     * @param array<string, mixed> $attributes
+     */
+    public function get(Model $model, string $key, mixed $value, array $attributes): ?Carbon
+    {
+        if('' === $value || null === $value) {
+            return null;
+        }
+        $timeZone = $attributes[sprintf('%s_tz', $key)] ?? config('app.timezone');
+        return Carbon::parse($value, $timeZone)->setTimezone(config('app.timezone'));
+    }
+
+    /**
+     * Prepare the given value for storage.
+     *
+     * @param array<string, mixed> $attributes
+     */
+    public function set(Model $model, string $key, mixed $value, array $attributes): mixed
+    {
+        return $value;
+    }
+}