diff --git a/custom_components/tibber_prices/coordinator/listeners.py b/custom_components/tibber_prices/coordinator/listeners.py
index 3791eee..41f88e9 100644
--- a/custom_components/tibber_prices/coordinator/listeners.py
+++ b/custom_components/tibber_prices/coordinator/listeners.py
@@ -157,7 +157,18 @@ class TibberPricesListenerManager:
         self,
         handler_callback: Callable[[datetime], None],
     ) -> None:
-        """Schedule 30-second entity refresh for timing sensors."""
+        """
+        Schedule 30-second entity refresh for timing sensors (Timer #3).
+
+        This is Timer #3 in the integration's timer architecture. It MUST trigger
+        at exact 30-second boundaries (0, 30 seconds) to keep timing sensors
+        (countdown, time-to) accurate.
+
+        Home Assistant may introduce small scheduling delays (jitter), which are
+        corrected using _BOUNDARY_TOLERANCE_SECONDS in time_service.py.
+
+        Runs independently of Timer #1 (API polling), which operates at random offsets.
+        """
         # Cancel any existing timer
         if self._minute_timer_cancel:
             self._minute_timer_cancel()
diff --git a/custom_components/tibber_prices/coordinator/midnight_handler.py b/custom_components/tibber_prices/coordinator/midnight_handler.py
new file mode 100644
index 0000000..159bfb3
--- /dev/null
+++ b/custom_components/tibber_prices/coordinator/midnight_handler.py
@@ -0,0 +1,121 @@
+"""
+Midnight turnover detection and coordination handler.
+
+This module provides atomic coordination logic for midnight turnover between
+multiple timers (DataUpdateCoordinator and quarter-hour refresh timer).
+
+The handler ensures that midnight turnover happens exactly once per day,
+regardless of which timer detects it first.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from datetime import datetime
+
+
+class TibberPricesMidnightHandler:
+    """
+    Handles midnight turnover detection and atomic coordination.
+
+    This class encapsulates the logic for detecting when midnight has passed
+    and ensuring that data rotation happens exactly once per day.
+
+    The atomic coordination works without locks by comparing date values:
+    - Timer #1 and Timer #2 both check if current_date > last_checked_date
+    - First timer to succeed marks the date as checked
+    - Second timer sees dates are equal and skips turnover
+    - Timer #3 doesn't participate in midnight logic (only 30-second timing updates)
+
+    HA Restart Handling:
+    - If HA restarts after midnight, _last_midnight_check is None (fresh handler)
+    - But _last_actual_turnover is restored from cache with yesterday's date
+    - is_turnover_needed() detects the date mismatch and returns True
+    - Missed midnight turnover is caught up on first timer run after restart
+
+    Attributes:
+        _last_midnight_check: Last datetime when midnight turnover was checked
+        _last_actual_turnover: Last datetime when turnover actually happened
+
+    """
+
+    def __init__(self) -> None:
+        """Initialize the midnight handler."""
+        self._last_midnight_check: datetime | None = None
+        self._last_actual_turnover: datetime | None = None
+
+    def is_turnover_needed(self, now: datetime) -> bool:
+        """
+        Check if midnight turnover is needed without side effects.
+
+        This is a pure check function - it doesn't modify state. Call
+        mark_turnover_done() after successfully performing the turnover.
+
+        IMPORTANT: If handler is uninitialized (HA restart), this checks if we
+        need to catch up on midnight turnover that happened while HA was down.
+
+        Args:
+            now: Current datetime to check
+
+        Returns:
+            True if midnight has passed since last check, False otherwise
+
+        """
+        # First time initialization after HA restart
+        if self._last_midnight_check is None:
+            # Check if we need to catch up on missed midnight turnover
+            # If last_actual_turnover exists, we can determine if midnight was missed
+            if self._last_actual_turnover is not None:
+                last_turnover_date = self._last_actual_turnover.date()
+                current_date = now.date()
+                # Turnover needed if we're on a different day than last turnover
+                return current_date > last_turnover_date
+            # Both None = fresh start, no turnover needed yet
+            return False
+
+        # Extract date components
+        last_checked_date = self._last_midnight_check.date()
+        current_date = now.date()
+
+        # Midnight crossed if current date is after last checked date
+        return current_date > last_checked_date
+
+    def mark_turnover_done(self, now: datetime) -> None:
+        """
+        Mark that midnight turnover has been completed.
+
+        Updates both check timestamp and actual turnover timestamp to prevent
+        duplicate turnover by another timer.
+
+        Args:
+            now: Current datetime when turnover was completed
+
+        """
+        self._last_midnight_check = now
+        self._last_actual_turnover = now
+
+    def update_check_time(self, now: datetime) -> None:
+        """
+        Update the last check time without marking turnover as done.
+
+        Used for initializing the handler or updating the check timestamp
+        without triggering turnover logic.
+
+        Args:
+            now: Current datetime to set as last check time
+
+        """
+        if self._last_midnight_check is None:
+            self._last_midnight_check = now
+
+    @property
+    def last_turnover_time(self) -> datetime | None:
+        """Get the timestamp of the last actual turnover."""
+        return self._last_actual_turnover
+
+    @property
+    def last_check_time(self) -> datetime | None:
+        """Get the timestamp of the last midnight check."""
+        return self._last_midnight_check
diff --git a/docs/development/critical-patterns.md b/docs/development/critical-patterns.md
new file mode 100644
index 0000000..4e3877d
--- /dev/null
+++ b/docs/development/critical-patterns.md
@@ -0,0 +1,286 @@
+# Critical Behavior Patterns - Testing Guide
+
+**Purpose:** This documentation lists essential behavior patterns that must be tested to ensure production-quality code and prevent resource leaks.
+
+**Last Updated:** 2025-11-22
+**Test Coverage:** 41 tests implemented (100% of critical patterns)
+
+## 🎯 Why Are These Tests Critical?
+
+Home Assistant integrations run **continuously** in the background. Resource leaks lead to:
+- **Memory Leaks**: RAM usage grows over days/weeks until HA becomes unstable
+- **Callback Leaks**: Listeners remain registered after entity removal → CPU load increases
+- **Timer Leaks**: Timers continue running after unload → unnecessary background tasks
+- **File Handle Leaks**: Storage files remain open → system resources exhausted
+
+## ✅ Test Categories
+
+### 1. Resource Cleanup (Memory Leak Prevention)
+
+**File:** `tests/test_resource_cleanup.py`
+
+#### 1.1 Listener Cleanup ✅
+
+**What is tested:**
+- Time-sensitive listeners are correctly removed (`async_add_time_sensitive_listener()`)
+- Minute-update listeners are correctly removed (`async_add_minute_update_listener()`)
+- Lifecycle callbacks are correctly unregistered (`register_lifecycle_callback()`)
+- Sensor cleanup removes ALL registered listeners
+- Binary sensor cleanup removes ALL registered listeners
+
+**Why critical:**
+- Each registered listener holds references to Entity + Coordinator
+- Without cleanup: Entities are not freed by GC → Memory Leak
+- With 80+ sensors × 3 listener types = 240+ callbacks that must be cleanly removed
+
+**Code Locations:**
+- `coordinator/listeners.py` → `async_add_time_sensitive_listener()`, `async_add_minute_update_listener()`
+- `coordinator/core.py` → `register_lifecycle_callback()`
+- `sensor/core.py` → `async_will_remove_from_hass()`
+- `binary_sensor/core.py` → `async_will_remove_from_hass()`
+
+#### 1.2 Timer Cleanup ✅
+
+**What is tested:**
+- Quarter-hour timer is cancelled and reference cleared
+- Minute timer is cancelled and reference cleared
+- Both timers are cancelled together
+- Cleanup works even when timers are `None`
+
+**Why critical:**
+- Uncancelled timers continue running after integration unload
+- HA's `async_track_utc_time_change()` creates persistent callbacks
+- Without cleanup: Timers keep firing → CPU load + unnecessary coordinator updates
+
+**Code Locations:**
+- `coordinator/listeners.py` → `cancel_timers()`
+- `coordinator/core.py` → `async_shutdown()`
+
+#### 1.3 Config Entry Cleanup ✅
+
+**What is tested:**
+- Options update listener is registered via `async_on_unload()`
+- Cleanup function is correctly passed to `async_on_unload()`
+
+**Why critical:**
+- `entry.add_update_listener()` registers permanent callback
+- Without `async_on_unload()`: Listener remains active after reload → duplicate updates
+- Pattern: `entry.async_on_unload(entry.add_update_listener(handler))`
+
+**Code Locations:**
+- `coordinator/core.py` → `__init__()` (listener registration)
+- `__init__.py` → `async_unload_entry()`
+
+### 2. Cache Invalidation ✅
+
+**File:** `tests/test_resource_cleanup.py`
+
+#### 2.1 Config Cache Invalidation
+
+**What is tested:**
+- DataTransformer config cache is invalidated on options change
+- PeriodCalculator config + period cache is invalidated
+- Trend calculator cache is cleared on coordinator update
+
+**Why critical:**
+- Stale config → Sensors use old user settings
+- Stale period cache → Incorrect best/peak price periods
+- Stale trend cache → Outdated trend analysis
+
+**Code Locations:**
+- `coordinator/data_transformation.py` → `invalidate_config_cache()`
+- `coordinator/periods.py` → `invalidate_config_cache()`
+- `sensor/calculators/trend.py` → `clear_trend_cache()`
+
+### 3. Storage Cleanup ✅
+
+**File:** `tests/test_resource_cleanup.py` + `tests/test_coordinator_shutdown.py`
+
+#### 3.1 Persistent Storage Removal
+
+**What is tested:**
+- Storage file is deleted on config entry removal
+- Cache is saved on shutdown (no data loss)
+
+**Why critical:**
+- Without storage removal: Old files remain after uninstallation
+- Without cache save on shutdown: Data loss on HA restart
+- Storage path: `.storage/tibber_prices.{entry_id}`
+
+**Code Locations:**
+- `__init__.py` → `async_remove_entry()`
+- `coordinator/core.py` → `async_shutdown()`
+
+### 4. Timer Scheduling ✅
+
+**File:** `tests/test_timer_scheduling.py`
+
+**What is tested:**
+- Quarter-hour timer is registered with correct parameters
+- Minute timer is registered with correct parameters
+- Timers can be re-scheduled (override old timer)
+- Midnight turnover detection works correctly
+
+**Why critical:**
+- Wrong timer parameters → Entities update at wrong times
+- Without timer override on re-schedule → Multiple parallel timers → Performance problem
+
+### 5. Sensor-to-Timer Assignment ✅
+
+**File:** `tests/test_sensor_timer_assignment.py`
+
+**What is tested:**
+- All `TIME_SENSITIVE_ENTITY_KEYS` are valid entity keys
+- All `MINUTE_UPDATE_ENTITY_KEYS` are valid entity keys
+- Both lists are disjoint (no overlap)
+- Sensor and binary sensor platforms are checked
+
+**Why critical:**
+- Wrong timer assignment → Sensors update at wrong times
+- Overlap → Duplicate updates → Performance problem
+
+## 🚨 Additional Analysis (Nice-to-Have Patterns)
+
+These patterns were analyzed and classified as **not critical**:
+
+### 6. Async Task Management
+
+**Current Status:** Fire-and-forget pattern for short tasks
+- `sensor/core.py` → Chart data refresh (short-lived, max 1-2 seconds)
+- `coordinator/core.py` → Cache storage (short-lived, max 100ms)
+
+**Why no tests needed:**
+- No long-running tasks (all < 2 seconds)
+- HA's event loop handles short tasks automatically
+- Task exceptions are already logged
+
+**If needed:** `_chart_refresh_task` tracking + cancel in `async_will_remove_from_hass()`
+
+### 7. API Session Cleanup
+
+**Current Status:** ✅ Correctly implemented
+- `async_get_clientsession(hass)` is used (shared session)
+- No new sessions are created
+- HA manages session lifecycle automatically
+
+**Code:** `api/client.py` + `__init__.py`
+
+### 8. Translation Cache Memory
+
+**Current Status:** ✅ Bounded cache
+- Max ~5-10 languages × 5KB = 50KB total
+- Module-level cache without re-loading
+- Practically no memory issue
+
+**Code:** `const.py` → `_TRANSLATIONS_CACHE`, `_STANDARD_TRANSLATIONS_CACHE`
+
+### 9. Coordinator Data Structure Integrity
+
+**Current Status:** Manually tested via `./scripts/develop`
+- Midnight turnover works correctly (observed over several days)
+- Missing keys are handled via `.get()` with defaults
+- 80+ sensors access `coordinator.data` without errors
+
+**Structure:**
+```python
+coordinator.data = {
+    "user_data": {...},
+    "priceInfo": {
+        "yesterday": [...],
+        "today": [...],
+        "tomorrow": [...],
+        "currency": "EUR"
+    }
+}
+```
+
+### 10. Service Response Memory
+
+**Current Status:** HA's response lifecycle
+- HA automatically frees service responses after return
+- ApexCharts ~20KB response is one-time per call
+- No response accumulation in integration code
+
+**Code:** `services/apexcharts.py`
+
+## 📊 Test Coverage Status
+
+### ✅ Implemented Tests (41 total)
+
+| Category | Status | Tests | File | Coverage |
+|----------|--------|-------|------|----------|
+| Listener Cleanup | ✅ | 5 | `test_resource_cleanup.py` | 100% |
+| Timer Cleanup | ✅ | 4 | `test_resource_cleanup.py` | 100% |
+| Config Entry Cleanup | ✅ | 1 | `test_resource_cleanup.py` | 100% |
+| Cache Invalidation | ✅ | 3 | `test_resource_cleanup.py` | 100% |
+| Storage Cleanup | ✅ | 1 | `test_resource_cleanup.py` | 100% |
+| Storage Persistence | ✅ | 2 | `test_coordinator_shutdown.py` | 100% |
+| Timer Scheduling | ✅ | 8 | `test_timer_scheduling.py` | 100% |
+| Sensor-Timer Assignment | ✅ | 17 | `test_sensor_timer_assignment.py` | 100% |
+| **TOTAL** | **✅** | **41** | | **100% (critical)** |
+
+### 📋 Analyzed but Not Implemented (Nice-to-Have)
+
+| Category | Status | Rationale |
+|----------|--------|-----------|
+| Async Task Management | 📋 | Fire-and-forget pattern used (no long-running tasks) |
+| API Session Cleanup | ✅ | Pattern correct (`async_get_clientsession` used) |
+| Translation Cache | ✅ | Cache size bounded (~50KB max for 10 languages) |
+| Data Structure Integrity | 📋 | Would add test time without finding real issues |
+| Service Response Memory | 📋 | HA automatically frees service responses |
+
+**Legend:**
+- ✅ = Fully tested or pattern verified correct
+- 📋 = Analyzed, low priority for testing (no known issues)
+
+## 🎯 Development Status
+
+### ✅ All Critical Patterns Tested
+
+All essential memory leak prevention patterns are covered by 41 tests:
+- ✅ Listeners are correctly removed (no callback leaks)
+- ✅ Timers are cancelled (no background task leaks)
+- ✅ Config entry cleanup works (no dangling listeners)
+- ✅ Caches are invalidated (no stale data issues)
+- ✅ Storage is saved and cleaned up (no data loss)
+- ✅ Timer scheduling works correctly (no update issues)
+- ✅ Sensor-timer assignment is correct (no wrong updates)
+
+### 📋 Nice-to-Have Tests (Optional)
+
+If problems arise in the future, these tests can be added:
+
+1. **Async Task Management** - Pattern analyzed (fire-and-forget for short tasks)
+2. **Data Structure Integrity** - Midnight rotation manually tested
+3. **Service Response Memory** - HA's response lifecycle automatic
+
+**Conclusion:** The integration has production-quality test coverage for all critical resource leak patterns.
+
+## 🔍 How to Run Tests
+
+```bash
+# Run all resource cleanup tests (14 tests)
+./scripts/test tests/test_resource_cleanup.py -v
+
+# Run all critical pattern tests (41 tests)
+./scripts/test tests/test_resource_cleanup.py tests/test_coordinator_shutdown.py \
+              tests/test_timer_scheduling.py tests/test_sensor_timer_assignment.py -v
+
+# Run all tests with coverage
+./scripts/test --cov=custom_components.tibber_prices --cov-report=html
+
+# Type checking and linting
+./scripts/check
+
+# Manual memory leak test
+# 1. Start HA: ./scripts/develop
+# 2. Monitor RAM: watch -n 1 'ps aux | grep home-assistant'
+# 3. Reload integration multiple times (HA UI: Settings → Devices → Tibber Prices → Reload)
+# 4. RAM should stabilize (not grow continuously)
+```
+
+## 📚 References
+
+- **Home Assistant Cleanup Patterns**: https://developers.home-assistant.io/docs/integration_setup_failures/#cleanup
+- **Async Best Practices**: https://developers.home-assistant.io/docs/asyncio_101/
+- **Memory Profiling**: https://docs.python.org/3/library/tracemalloc.html
diff --git a/tests/test_midnight_handler.py b/tests/test_midnight_handler.py
new file mode 100644
index 0000000..52ae086
--- /dev/null
+++ b/tests/test_midnight_handler.py
@@ -0,0 +1,322 @@
+"""
+Unit tests for midnight turnover handler.
+
+These tests verify the atomic coordination logic that prevents duplicate
+midnight turnover between multiple timers.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime
+from zoneinfo import ZoneInfo
+
+import pytest
+
+from custom_components.tibber_prices.coordinator.midnight_handler import (
+    TibberPricesMidnightHandler,
+)
+
+
+@pytest.mark.unit
+def test_first_check_initializes_without_turnover() -> None:
+    """Test that the first check initializes but doesn't trigger turnover."""
+    handler = TibberPricesMidnightHandler()
+
+    time1 = datetime(2025, 11, 22, 14, 30, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+
+    # First check should return False (no turnover yet)
+    assert not handler.is_turnover_needed(time1)
+
+    # But update_check_time should initialize
+    handler.update_check_time(time1)
+    assert handler.last_check_time == time1
+
+
+@pytest.mark.unit
+def test_midnight_crossing_triggers_turnover() -> None:
+    """Test that crossing midnight triggers turnover detection."""
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize at 23:59:59 on Nov 22
+    time1 = datetime(2025, 11, 22, 23, 59, 59, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    # Check at 00:00:00 on Nov 23 (midnight crossed!)
+    time2 = datetime(2025, 11, 23, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(time2)
+
+
+@pytest.mark.unit
+def test_same_day_no_turnover() -> None:
+    """Test that multiple checks on the same day don't trigger turnover."""
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize at 10:00
+    time1 = datetime(2025, 11, 22, 10, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    # Check later same day at 14:00
+    time2 = datetime(2025, 11, 22, 14, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert not handler.is_turnover_needed(time2)
+
+    # Check even later at 23:59
+    time3 = datetime(2025, 11, 22, 23, 59, 59, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert not handler.is_turnover_needed(time3)
+
+
+@pytest.mark.unit
+def test_atomic_coordination_prevents_duplicate_turnover() -> None:
+    """Test that marking turnover done prevents duplicate execution."""
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize on Nov 22
+    time1 = datetime(2025, 11, 22, 23, 50, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    # Midnight on Nov 23 - first timer detects it
+    midnight = datetime(2025, 11, 23, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(midnight)
+
+    # First timer marks it done
+    handler.mark_turnover_done(midnight)
+
+    # Second timer checks shortly after - should return False
+    time2 = datetime(2025, 11, 23, 0, 0, 10, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert not handler.is_turnover_needed(time2)
+
+
+@pytest.mark.unit
+def test_mark_turnover_updates_both_timestamps() -> None:
+    """Test that mark_turnover_done updates both check and turnover timestamps."""
+    handler = TibberPricesMidnightHandler()
+
+    time1 = datetime(2025, 11, 22, 10, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    midnight = datetime(2025, 11, 23, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.mark_turnover_done(midnight)
+
+    # Both timestamps should be updated
+    assert handler.last_check_time == midnight
+    assert handler.last_turnover_time == midnight
+
+
+@pytest.mark.unit
+def test_next_day_triggers_new_turnover() -> None:
+    """Test that the next day's midnight triggers turnover again."""
+    handler = TibberPricesMidnightHandler()
+
+    # Day 1: Initialize and mark turnover done
+    day1 = datetime(2025, 11, 22, 10, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(day1)
+
+    midnight1 = datetime(2025, 11, 23, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(midnight1)
+    handler.mark_turnover_done(midnight1)
+
+    # Day 2: Next midnight should trigger again
+    midnight2 = datetime(2025, 11, 24, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(midnight2)
+
+
+@pytest.mark.unit
+def test_multiple_days_skipped_still_triggers() -> None:
+    """Test that skipping multiple days still triggers turnover."""
+    handler = TibberPricesMidnightHandler()
+
+    # Last check on Nov 22
+    time1 = datetime(2025, 11, 22, 10, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    # Check 3 days later on Nov 25 (skipped 23rd and 24th)
+    time2 = datetime(2025, 11, 25, 14, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(time2)
+
+
+@pytest.mark.unit
+def test_update_check_time_without_triggering_turnover() -> None:
+    """Test that update_check_time initializes without turnover side effects."""
+    handler = TibberPricesMidnightHandler()
+
+    time1 = datetime(2025, 11, 22, 10, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(time1)
+
+    # Last turnover should still be None
+    assert handler.last_check_time == time1
+    assert handler.last_turnover_time is None
+
+    # Next day should trigger turnover
+    time2 = datetime(2025, 11, 23, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(time2)
+
+
+@pytest.mark.unit
+def test_ha_restart_after_midnight_with_cached_turnover() -> None:
+    """
+    Test HA restart scenario: cached turnover from yesterday, restart after midnight.
+
+    Scenario:
+    - Nov 21 23:50: Last turnover marked (before HA shutdown)
+    - Nov 22 00:30: HA restarts (handler is fresh, but turnover was cached)
+    - Expected: Turnover should be triggered to catch up
+
+    This simulates: mark_turnover_done() was called on Nov 21, handler state is
+    restored (simulated by manually setting _last_actual_turnover), then first
+    check after restart should detect missed midnight.
+    """
+    handler = TibberPricesMidnightHandler()
+
+    # Simulate: Last turnover was on Nov 21 at 23:59:59 (just before midnight)
+    last_turnover = datetime(2025, 11, 21, 23, 59, 59, tzinfo=ZoneInfo("Europe/Oslo"))
+    # Manually restore handler state (simulates cache restoration)
+    handler._last_actual_turnover = last_turnover  # noqa: SLF001 - Test setup
+
+    # HA restarts at Nov 22 00:30 (after midnight)
+    restart_time = datetime(2025, 11, 22, 0, 30, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+
+    # First check after restart - should detect missed midnight
+    # _last_midnight_check is None (fresh handler), but _last_actual_turnover exists
+    assert handler.is_turnover_needed(restart_time) is True
+
+    # Perform turnover
+    handler.mark_turnover_done(restart_time)
+
+    # Second check - should not trigger again
+    time_2 = datetime(2025, 11, 22, 1, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(time_2) is False
+
+
+@pytest.mark.unit
+def test_ha_restart_same_day_with_cached_turnover() -> None:
+    """
+    Test HA restart scenario: cached turnover from today, restart same day.
+
+    Scenario:
+    - Nov 22 00:05: Turnover happened (after HA started)
+    - Nov 22 14:00: HA restarts
+    - Expected: No turnover needed (already done today)
+
+    This ensures we don't trigger duplicate turnover when restarting same day.
+    """
+    handler = TibberPricesMidnightHandler()
+
+    # Simulate: Last turnover was today at 00:05
+    last_turnover = datetime(2025, 11, 22, 0, 5, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    # Manually restore handler state (simulates cache restoration)
+    handler._last_actual_turnover = last_turnover  # noqa: SLF001 - Test setup
+
+    # HA restarts at 14:00 same day
+    restart_time = datetime(2025, 11, 22, 14, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+
+    # First check after restart - should NOT trigger (same day)
+    assert handler.is_turnover_needed(restart_time) is False
+
+    # Initialize check time for subsequent checks
+    handler.update_check_time(restart_time)
+
+    # Later check same day - still no turnover
+    time_2 = datetime(2025, 11, 22, 18, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(time_2) is False
+
+
+@pytest.mark.unit
+def test_simultaneous_timer_checks_at_midnight() -> None:
+    """
+    Test race condition: Timer #1 and Timer #2 both check at exactly 00:00:00.
+
+    This is the critical atomic coordination test - both timers detect midnight
+    simultaneously, but only one should perform turnover.
+
+    Scenario:
+    - Nov 21 23:45: Both timers initialized
+    - Nov 22 00:00:00: Both timers check simultaneously
+    - Expected: First check returns True, second returns False (atomic)
+    """
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize on Nov 21 at 23:45
+    init_time = datetime(2025, 11, 21, 23, 45, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(init_time)
+
+    # Both timers wake up at exactly 00:00:00
+    midnight = datetime(2025, 11, 22, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+
+    # Timer #1 checks first (or both check "simultaneously" in sequence)
+    timer1_check = handler.is_turnover_needed(midnight)
+    assert timer1_check is True  # Midnight crossed
+
+    # Timer #1 performs turnover
+    handler.mark_turnover_done(midnight)
+
+    # Timer #2 checks immediately after (could be microseconds later)
+    timer2_check = handler.is_turnover_needed(midnight)
+    assert timer2_check is False  # Already done by Timer #1
+
+    # Verify state: turnover happened exactly once
+    assert handler.last_turnover_time == midnight
+    assert handler.last_check_time == midnight
+
+
+@pytest.mark.unit
+def test_timer_check_at_00_00_01_after_turnover_at_00_00_00() -> None:
+    """
+    Test edge case: One timer does turnover at 00:00:00, second checks at 00:00:01.
+
+    This ensures that even a 1-second delay doesn't cause duplicate turnover
+    when both checks happen on the same calendar day.
+
+    Scenario:
+    - Nov 22 00:00:00: Timer #1 does turnover
+    - Nov 22 00:00:01: Timer #2 checks (1 second later)
+    - Expected: Timer #2 should skip (same day)
+    """
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize on Nov 21
+    init_time = datetime(2025, 11, 21, 23, 45, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(init_time)
+
+    # Timer #1 checks at exactly 00:00:00
+    midnight_00 = datetime(2025, 11, 22, 0, 0, 0, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(midnight_00) is True
+    handler.mark_turnover_done(midnight_00)
+
+    # Timer #2 checks 1 second later
+    midnight_01 = datetime(2025, 11, 22, 0, 0, 1, tzinfo=ZoneInfo("Europe/Oslo"))
+    assert handler.is_turnover_needed(midnight_01) is False
+
+    # Both timestamps point to same day - no duplicate
+    assert handler.last_turnover_time.date() == midnight_01.date()  # type: ignore[union-attr]
+
+
+@pytest.mark.unit
+def test_rapid_consecutive_checks_same_second() -> None:
+    """
+    Test rapid consecutive checks within the same second at midnight.
+
+    Simulates worst-case race condition where both timers fire within
+    the same second (e.g., 00:00:00.123 and 00:00:00.456).
+
+    Expected: First check triggers, all subsequent checks skip.
+    """
+    handler = TibberPricesMidnightHandler()
+
+    # Initialize on Nov 21
+    init_time = datetime(2025, 11, 21, 23, 59, 59, tzinfo=ZoneInfo("Europe/Oslo"))
+    handler.update_check_time(init_time)
+
+    # Simulate 3 checks at midnight within the same second
+    midnight_check1 = datetime(2025, 11, 22, 0, 0, 0, 123000, tzinfo=ZoneInfo("Europe/Oslo"))
+    midnight_check2 = datetime(2025, 11, 22, 0, 0, 0, 456000, tzinfo=ZoneInfo("Europe/Oslo"))
+    midnight_check3 = datetime(2025, 11, 22, 0, 0, 0, 789000, tzinfo=ZoneInfo("Europe/Oslo"))
+
+    # First check: turnover needed
+    assert handler.is_turnover_needed(midnight_check1) is True
+    handler.mark_turnover_done(midnight_check1)
+
+    # Second and third checks: already done
+    assert handler.is_turnover_needed(midnight_check2) is False
+    assert handler.is_turnover_needed(midnight_check3) is False
+
+    # Verify: turnover happened exactly once
+    assert handler.last_turnover_time == midnight_check1