test: add missing return type annotations for timezone helper functions

- Add explicit return types to isEuropeBerlinTimezone and maybeSkipTimezoneDependent functions
- Remove unused import dateStrToUtcDate from get-next-repeat-occurrence.util.spec.ts
- Resolves TypeScript lint errors for explicit-function-return-type and no-unused-vars rules

src/app/features/schedule/map-schedule-data/create-schedule-view-entries-for-normal-tasks.spec.ts

@@ -17,8 +17,8 @@ const hours = (n: number): number => 60 * minutes(n);
 // Helper function to conditionally skip tests that are timezone-dependent
 // These tests were written with hardcoded expectations for Europe/Berlin timezone
 const TZ_OFFSET = new Date('1970-01-01').getTimezoneOffset() * 60000;
-const isEuropeBerlinTimezone = () => TZ_OFFSET === -3600000; // UTC+1 = -1 hour offset
-const maybeSkipTimezoneDependent = (testName: string) => {
+const isEuropeBerlinTimezone = (): boolean => TZ_OFFSET === -3600000; // UTC+1 = -1 hour offset
+const maybeSkipTimezoneDependent = (testName: string): boolean => {
   if (!isEuropeBerlinTimezone()) {
     console.warn(
       `Skipping timezone-dependent test "${testName}" - only runs in Europe/Berlin timezone`,

src/app/features/task-repeat-cfg/store/get-next-repeat-occurrence.util.spec.ts

@@ -1,7 +1,6 @@
 import { getNextRepeatOccurrence } from './get-next-repeat-occurrence.util';
 import { DEFAULT_TASK_REPEAT_CFG, TaskRepeatCfg } from '../task-repeat-cfg.model';
 import { getLocalDateStr } from '../../../util/get-local-date-str';
-import { dateStrToUtcDate } from '../../../util/date-str-to-utc-date';
 
 /* eslint-disable no-mixed-operators */
 

timezone-test-failures-summary.md

@@ -0,0 +1,115 @@
+# Timezone Test Failures in LA Timezone
+
+## Root Cause
+
+When `karma.conf.js` line 7 is commented out (removing `process.env.TZ = 'Europe/Berlin'`), tests fail in LA timezone due to date parsing issues.
+
+## Main Issues
+
+### 1. Date Constructor Ambiguity
+
+**Problem**: `new Date('2020-07-06')` creates different dates in different timezones:
+
+- **Berlin**: July 6, 2020 at 00:00 local time
+- **LA**: July 5, 2020 at 17:00 (5 PM) local time (UTC interprets as July 6 00:00, then converts to LA time)
+
+**Affected Tests**:
+
+- `get-week-number.spec.ts`: Expects week 28 but gets week 27
+- `get-date-range-for-week.spec.ts`: Week boundaries are off by one day
+
+### 2. DST (Daylight Saving Time) Issues
+
+**Problem**: Tests expecting 24-hour days fail during DST transitions:
+
+- Spring forward: Day is 23 hours
+- Fall back: Day is 25 hours
+
+**Affected Tests**:
+
+- `get-date-range-for-day.spec.ts`: DST transition tests
+
+### 3. Week Start/End Calculations
+
+**Problem**: Week boundary calculations assume consistent timezone behavior
+
+- Week calculations are off by one day when dates cross timezone boundaries
+
+**Affected Tests**:
+
+- `create-blocked-blocks-by-day-map.spec.ts`: Date keys in maps don't match
+
+## Recommended Fixes
+
+### Option 1: Keep Berlin Timezone for Tests (Current Approach)
+
+- **Pros**: All tests pass consistently, predictable behavior
+- **Cons**: Doesn't catch real timezone issues that users might face
+
+### Option 2: Fix Date Creation in Tests
+
+Replace ambiguous date constructors:
+
+```typescript
+// BAD - timezone dependent
+new Date('2020-07-06');
+
+// GOOD - explicit local time
+new Date(2020, 6, 6); // July 6, 2020 (month is 0-indexed)
+
+// GOOD - explicit UTC
+new Date(Date.UTC(2020, 6, 6));
+```
+
+### Option 3: Use UTC for All Test Dates
+
+Consistently use UTC dates in tests to avoid timezone issues:
+
+```typescript
+import { dateStrToUtcDate } from '../../../util/date-str-to-utc-date';
+const testDate = dateStrToUtcDate('2020-07-06');
+```
+
+### Option 4: Mock Date/Time in Tests
+
+Use a library like `@sinonjs/fake-timers` to control time in tests:
+
+```typescript
+import { install } from '@sinonjs/fake-timers';
+const clock = install({ now: new Date('2020-07-06T12:00:00Z') });
+// Run tests
+clock.uninstall();
+```
+
+## Tests That Need Fixing (Sample)
+
+1. **get-week-number.spec.ts**
+
+   - Line 5: `new Date('2020-07-06')` → `new Date(2020, 6, 6)`
+   - Line 11: `new Date('2020-01-01')` → `new Date(2020, 0, 1)`
+   - Line 17, 21: `new Date('2020-01-08')` → `new Date(2020, 0, 8)`
+   - Line 27: `new Date('2020-12-31')` → `new Date(2020, 11, 31)`
+   - Line 33: `new Date('2021-12-31')` → `new Date(2021, 11, 31)`
+
+2. **get-date-range-for-week.spec.ts**
+
+   - Line 11: `new Date('2020-07-06')` → `new Date(2020, 6, 6)`
+   - Line 12: `new Date('2020-07-12')` → `new Date(2020, 6, 12)`
+   - Line 17: `new Date('2020-12-28')` → `new Date(2020, 11, 28)`
+   - Line 18: `new Date('2021-01-03')` → `new Date(2021, 0, 3)`
+   - Line 23: `new Date('2021-01-04')` → `new Date(2021, 0, 4)`
+   - Line 24: `new Date('2021-01-10')` → `new Date(2021, 0, 10)`
+
+3. **Issue #4840 Tests** (our new tests)
+   - These correctly demonstrate the timezone-dependent behavior
+   - They SHOULD fail in LA to prove the bug exists
+
+## Conclusion
+
+The current approach (hardcoding Berlin timezone in karma.conf.js) ensures consistent test behavior but masks real timezone issues. To properly support users in all timezones, either:
+
+1. Keep the Berlin timezone for tests but add specific timezone edge case tests
+2. Fix all date creation in tests to use explicit local dates
+3. Run tests in multiple timezones in CI/CD to catch issues
+
+The issue #4840 we discovered is a real bug that affects users in different timezones differently, and our tests correctly identify it when run in LA timezone.