diff --git a/core/app/Models/Concerns/StoresUtc.php b/core/app/Models/Concerns/StoresUtc.php
new file mode 100644
index 0000000..7a08b04
--- /dev/null
+++ b/core/app/Models/Concerns/StoresUtc.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace App\Models\Concerns;
+
+use Carbon\CarbonImmutable;
+use DateTimeInterface;
+
+/**
+ * Eloquent форматирует datetime без смещения, поэтому Carbon в чужом поясе
+ * записался бы как «те же цифры в UTC». Приводим к UTC при присвоении.
+ */
+trait StoresUtc
+{
+    public function setAttribute($key, $value)
+    {
+        if ($value instanceof DateTimeInterface
+            && in_array($this->getCasts()[$key] ?? null, ['datetime', 'immutable_datetime'], true)) {
+            $value = CarbonImmutable::instance($value)->utc();
+        }
+
+        return parent::setAttribute($key, $value);
+    }
+}
diff --git a/core/app/Models/Delivery.php b/core/app/Models/Delivery.php
index 2f19e63..766bfd3 100644
--- a/core/app/Models/Delivery.php
+++ b/core/app/Models/Delivery.php
@@ -3,11 +3,14 @@
 namespace App\Models;
 
 use App\Enums\DeliveryResult;
+use App\Models\Concerns\StoresUtc;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 
 class Delivery extends Model
 {
+    use StoresUtc;
+
     protected $guarded = [];
 
     protected $attributes = ['meta' => '{}'];
diff --git a/core/app/Models/Event.php b/core/app/Models/Event.php
index d17c244..d74ee4b 100644
--- a/core/app/Models/Event.php
+++ b/core/app/Models/Event.php
@@ -5,12 +5,15 @@ namespace App\Models;
 use App\Enums\AfterDue;
 use App\Enums\DueMode;
 use App\Enums\EventState;
+use App\Models\Concerns\StoresUtc;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 
 class Event extends Model
 {
+    use StoresUtc;
+
     protected $guarded = [];
 
     protected function casts(): array
diff --git a/core/app/Scheduling/Cascade.php b/core/app/Scheduling/Cascade.php
new file mode 100644
index 0000000..61e58c1
--- /dev/null
+++ b/core/app/Scheduling/Cascade.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace App\Scheduling;
+
+use Carbon\CarbonImmutable;
+
+final class Cascade
+{
+    /**
+     * Точки каскада между стартом и днём срока: старт, затем каждый раз, когда
+     * до срока остаётся вдвое меньше дней, до одного дня. Обе даты — локальные
+     * полуночи в одном поясе. Возвращает локальные полуночи по возрастанию.
+     *
+     * @return list<CarbonImmutable>
+     */
+    public static function points(CarbonImmutable $fireStart, CarbonImmutable $dueDay): array
+    {
+        $d = (int) round($fireStart->diffInDays($dueDay));
+        $points = [];
+        while ($d >= 1) {
+            $points[] = $dueDay->subDays($d);
+            $d = intdiv($d, 2);
+        }
+
+        return $points;
+    }
+}
diff --git a/core/app/Scheduling/Due.php b/core/app/Scheduling/Due.php
new file mode 100644
index 0000000..5148de9
--- /dev/null
+++ b/core/app/Scheduling/Due.php
@@ -0,0 +1,16 @@
+<?php
+
+namespace App\Scheduling;
+
+use Carbon\CarbonImmutable;
+
+/** Срок события, приведённый к поясу пользователя. */
+final class Due
+{
+    public function __construct(
+        /** Локальная полночь дня срока. */
+        public readonly CarbonImmutable $day,
+        /** Момент срока. Для события «весь день» — следующая полночь. */
+        public readonly CarbonImmutable $moment,
+    ) {}
+}
diff --git a/core/app/Scheduling/DueResolver.php b/core/app/Scheduling/DueResolver.php
new file mode 100644
index 0000000..a2355fa
--- /dev/null
+++ b/core/app/Scheduling/DueResolver.php
@@ -0,0 +1,62 @@
+<?php
+
+namespace App\Scheduling;
+
+use App\Enums\DueMode;
+use App\Models\Event;
+use App\Models\User;
+use Carbon\CarbonImmutable;
+
+final class DueResolver
+{
+    public const WHOLE_DAY = '24:00:00';
+
+    public function resolve(Event $event, User $user): Due
+    {
+        return match ($event->due_mode) {
+            DueMode::Fixed => $this->fixed($event, $user),
+            DueMode::Local => $this->local($event, $user),
+        };
+    }
+
+    public function fireStart(Event $event, User $user): CarbonImmutable
+    {
+        return self::localMidnight($event->fire_on->format('Y-m-d'), $user->tz);
+    }
+
+    /**
+     * Старт дня срока. Обычно локальная полночь дня срока. Если момент срока не
+     * позже начала активных часов, первое окно дня уже опоздало бы — тогда старт
+     * за два часа до тихих часов накануне (два последних часовых окна вечера).
+     */
+    public function todayStart(Due $due, User $user): CarbonImmutable
+    {
+        if ($due->moment->lte($user->activeStart($due->day))) {
+            return $user->activeEnd($due->day->subDay())->subHours(2);
+        }
+
+        return $due->day;
+    }
+
+    public static function localMidnight(string $ymd, string $tz): CarbonImmutable
+    {
+        return CarbonImmutable::createFromFormat('Y-m-d', $ymd, $tz)->startOfDay();
+    }
+
+    private function local(Event $event, User $user): Due
+    {
+        $day = self::localMidnight($event->due_date->format('Y-m-d'), $user->tz);
+        $moment = $event->due_time === self::WHOLE_DAY
+            ? $day->addDay()
+            : $day->setTimeFromTimeString($event->due_time);
+
+        return new Due($day, $moment);
+    }
+
+    private function fixed(Event $event, User $user): Due
+    {
+        $moment = $event->due_instant->setTimezone($user->tz);
+
+        return new Due($moment->startOfDay(), $moment);
+    }
+}
diff --git a/core/tests/Feature/SchemaTest.php b/core/tests/Feature/SchemaTest.php
index 182c030..455bde0 100644
--- a/core/tests/Feature/SchemaTest.php
+++ b/core/tests/Feature/SchemaTest.php
@@ -75,6 +75,29 @@ class SchemaTest extends TestCase
         $this->assertSame(ChannelType::Web, $user->channels()->first()->type);
     }
 
+    public function test_datetimes_are_stored_as_utc(): void
+    {
+        $source = Source::create(['name' => 'docs', 'token_hash' => hash('sha256', 'x')]);
+        $user = User::create(['login' => 'nikita', 'tz' => 'Asia/Shanghai', 'quiet_start' => '22:00:00', 'quiet_end' => '09:00:00']);
+
+        $event = Event::create([
+            'source_id' => $source->id,
+            'user_id' => $user->id,
+            'source_ref' => 'person:42',
+            'topic' => 'birthday:2026',
+            'due_mode' => DueMode::Local,
+            'due_date' => '2026-08-14',
+            'due_time' => '24:00:00',
+            'fire_on' => '2026-08-14',
+            'after_due' => AfterDue::Expire,
+            'payload' => ['title' => 'ДР'],
+            'state' => EventState::Scheduled,
+            'quiet_until' => CarbonImmutable::parse('2026-09-10T10:00:00', 'Asia/Shanghai'),
+        ])->fresh();
+
+        $this->assertSame('2026-09-10T02:00:00+00:00', $event->quiet_until->utc()->toIso8601String());
+    }
+
     public function test_state_helpers(): void
     {
         $this->assertTrue(EventState::Done->isTerminal());
diff --git a/core/tests/Unit/Scheduling/CascadeTest.php b/core/tests/Unit/Scheduling/CascadeTest.php
new file mode 100644
index 0000000..dbb9d98
--- /dev/null
+++ b/core/tests/Unit/Scheduling/CascadeTest.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Tests\Unit\Scheduling;
+
+use App\Scheduling\Cascade;
+use Carbon\CarbonImmutable;
+use Tests\TestCase; // Laravel TestCase: Eloquent date-casts need the connection grammar even in memory
+
+class CascadeTest extends TestCase
+{
+    private function daysBefore(array $points, CarbonImmutable $due): array
+    {
+        return array_map(fn (CarbonImmutable $p) => (int) round($p->diffInDays($due)), $points);
+    }
+
+    public function test_no_preparation_gives_no_points(): void
+    {
+        $due = CarbonImmutable::parse('2026-08-14', 'UTC');
+        $this->assertSame([], Cascade::points($due, $due));
+    }
+
+    public function test_halving_points(): void
+    {
+        $due = CarbonImmutable::parse('2026-09-30', 'UTC');
+
+        $this->assertSame([1], $this->daysBefore(Cascade::points($due->subDay(), $due), $due));
+        $this->assertSame([7, 3, 1], $this->daysBefore(Cascade::points($due->subDays(7), $due), $due));
+        $this->assertSame([30, 15, 7, 3, 1], $this->daysBefore(Cascade::points($due->subDays(30), $due), $due));
+        $this->assertSame([60, 30, 15, 7, 3, 1], $this->daysBefore(Cascade::points($due->subDays(60), $due), $due));
+        $this->assertSame([180, 90, 45, 22, 11, 5, 2, 1], $this->daysBefore(Cascade::points($due->subDays(180), $due), $due));
+    }
+
+    public function test_points_are_local_midnights_ascending(): void
+    {
+        $due = CarbonImmutable::parse('2026-09-30', 'Asia/Shanghai');
+        $points = Cascade::points($due->subDays(7), $due);
+
+        $this->assertSame('2026-09-23 00:00 Asia/Shanghai', $points[0]->format('Y-m-d H:i e'));
+        $this->assertSame('2026-09-29 00:00 Asia/Shanghai', $points[2]->format('Y-m-d H:i e'));
+    }
+}
diff --git a/core/tests/Unit/Scheduling/DueResolverTest.php b/core/tests/Unit/Scheduling/DueResolverTest.php
new file mode 100644
index 0000000..4b5d9db
--- /dev/null
+++ b/core/tests/Unit/Scheduling/DueResolverTest.php
@@ -0,0 +1,99 @@
+<?php
+
+namespace Tests\Unit\Scheduling;
+
+use App\Enums\AfterDue;
+use App\Enums\DueMode;
+use App\Enums\EventState;
+use App\Models\Event;
+use App\Models\User;
+use App\Scheduling\DueResolver;
+use Carbon\CarbonImmutable;
+use Tests\TestCase; // Laravel TestCase: Eloquent date-casts need the connection grammar even in memory
+
+class DueResolverTest extends TestCase
+{
+    private function user(string $tz = 'America/Montevideo'): User
+    {
+        return new User(['login' => 'n', 'tz' => $tz, 'quiet_start' => '22:00:00', 'quiet_end' => '09:00:00']);
+    }
+
+    private function local(string $date, string $time = '24:00:00', ?string $fireOn = null): Event
+    {
+        return new Event([
+            'due_mode' => DueMode::Local, 'due_date' => $date, 'due_time' => $time,
+            'fire_on' => $fireOn ?? $date, 'after_due' => AfterDue::Expire,
+            'state' => EventState::Scheduled, 'payload' => ['title' => 't'],
+        ]);
+    }
+
+    private function fixed(string $instant, string $fireOn): Event
+    {
+        return new Event([
+            'due_mode' => DueMode::Fixed, 'due_instant' => CarbonImmutable::parse($instant),
+            'fire_on' => $fireOn, 'after_due' => AfterDue::Expire,
+            'state' => EventState::Scheduled, 'payload' => ['title' => 't'],
+        ]);
+    }
+
+    public function test_whole_day_local_event(): void
+    {
+        $due = (new DueResolver)->resolve($this->local('2026-08-14'), $this->user());
+
+        $this->assertSame('2026-08-14 00:00 America/Montevideo', $due->day->format('Y-m-d H:i e'));
+        $this->assertSame('2026-08-15 00:00 America/Montevideo', $due->moment->format('Y-m-d H:i e'));
+    }
+
+    public function test_timed_local_event(): void
+    {
+        $due = (new DueResolver)->resolve($this->local('2026-09-10', '14:00:00'), $this->user());
+
+        $this->assertSame('2026-09-10 00:00', $due->day->format('Y-m-d H:i'));
+        $this->assertSame('2026-09-10 14:00', $due->moment->format('Y-m-d H:i'));
+    }
+
+    public function test_local_event_follows_user_timezone(): void
+    {
+        $event = $this->local('2026-09-10', '14:00:00');
+
+        $uy = (new DueResolver)->resolve($event, $this->user('America/Montevideo'));
+        $cn = (new DueResolver)->resolve($event, $this->user('Asia/Shanghai'));
+
+        $this->assertSame('2026-09-10 14:00', $uy->moment->format('Y-m-d H:i'));
+        $this->assertSame('2026-09-10 14:00', $cn->moment->format('Y-m-d H:i'));
+        $this->assertNotEquals($uy->moment->utc()->toIso8601String(), $cn->moment->utc()->toIso8601String());
+    }
+
+    public function test_fixed_event_is_converted_into_user_timezone(): void
+    {
+        // Вылет из Мадрида 08:15+02:00 = 06:15Z = 03:15 в Монтевидео (-03:00)
+        $due = (new DueResolver)->resolve($this->fixed('2026-09-10T08:15:00+02:00', '2026-09-03'), $this->user());
+
+        $this->assertSame('2026-09-10 03:15 America/Montevideo', $due->moment->format('Y-m-d H:i e'));
+        $this->assertSame('2026-09-10 00:00', $due->day->format('Y-m-d H:i'));
+    }
+
+    public function test_fire_start_is_local_midnight(): void
+    {
+        $start = (new DueResolver)->fireStart($this->local('2026-09-30', '24:00:00', '2026-08-31'), $this->user());
+        $this->assertSame('2026-08-31 00:00 America/Montevideo', $start->format('Y-m-d H:i e'));
+    }
+
+    public function test_today_starts_at_midnight_for_normal_moment(): void
+    {
+        $r = new DueResolver;
+        $due = $r->resolve($this->local('2026-09-10', '14:00:00'), $this->user());
+        $this->assertSame('2026-09-10 00:00', $r->todayStart($due, $this->user())->format('Y-m-d H:i'));
+    }
+
+    public function test_today_starts_the_evening_before_for_early_moment(): void
+    {
+        $r = new DueResolver;
+        $due = $r->resolve($this->local('2026-09-10', '07:00:00'), $this->user());
+        // quiet_start 22:00 − 2h = 20:00 накануне
+        $this->assertSame('2026-09-09 20:00', $r->todayStart($due, $this->user())->format('Y-m-d H:i'));
+
+        $exactlyAtActiveStart = $r->resolve($this->local('2026-09-10', '09:00:00'), $this->user());
+        $this->assertSame('2026-09-09 20:00', $r->todayStart($exactlyAtActiveStart, $this->user())->format('Y-m-d H:i'));
+    }
+}