fix(schedule): fix timezone issues when parsing ISO date strings

Created parseDbDateStr utility to parse 'YYYY-MM-DD' strings as local
dates instead of UTC. Using new Date('2026-01-12') parses as UTC
midnight, which becomes the previous day when converted to timezones
like Europe/Berlin (UTC+1) or America/Los_Angeles (UTC-8).

Fixed in:
- ScheduleService.getDayClass
- ScheduleMonthComponent.referenceMonth
- ScheduleWeekComponent._formatDateLabel
- Test expectations in multiple spec files

All tests now pass in both Europe/Berlin and America/Los_Angeles timezones.

src/app/features/schedule/map-schedule-data/create-schedule-days.spec.ts

@@ -2,6 +2,7 @@ import { createScheduleDays } from './create-schedule-days';
 import { DEFAULT_TASK, TaskWithoutReminder } from '../../tasks/task.model';
 import { PlannerDayMap } from '../../planner/planner.model';
 import { BlockedBlockByDayMap } from '../schedule.model';
+import { parseDbDateStr } from '../../../util/parse-db-date-str';
 
 // Helper function to create test tasks with required properties
 const createTestTask = (
@@ -386,7 +387,7 @@ describe('createScheduleDays - Task Filtering', () => {
       });
 
       // Viewing two days in next week
-      const secondDayNextWeek = new Date(nextWeekStr);
+      const secondDayNextWeek = parseDbDateStr(nextWeekStr);
       secondDayNextWeek.setDate(secondDayNextWeek.getDate() + 1);
       const secondDayStr = secondDayNextWeek.toISOString().split('T')[0];
 
@@ -451,7 +452,7 @@ describe('createScheduleDays - Task Filtering', () => {
   describe('Tasks with dueWithTime', () => {
     it('should appear when viewing a week that includes the dueWithTime', () => {
       // Arrange
-      const nextWeekDate = new Date(nextWeekStr);
+      const nextWeekDate = parseDbDateStr(nextWeekStr);
       nextWeekDate.setHours(14, 0, 0, 0); // 2 PM next week
       const taskWithDueTime = createTestTask('task13', 'Task With Due Time', {
         dueWithTime: nextWeekDate.getTime(),
@@ -482,7 +483,7 @@ describe('createScheduleDays - Task Filtering', () => {
 
     it('should NOT appear when dueWithTime is before the viewed week', () => {
       // Arrange
-      const todayDate = new Date(todayStr);
+      const todayDate = parseDbDateStr(todayStr);
       todayDate.setHours(14, 0, 0, 0);
       const taskWithDueTime = createTestTask('task14', 'Task With Due Time Today', {
         dueWithTime: todayDate.getTime(),

src/app/features/schedule/schedule-month/schedule-month.component.spec.ts

@@ -2,6 +2,7 @@ import { ComponentFixture, TestBed } from '@angular/core/testing';
 import { ScheduleMonthComponent } from './schedule-month.component';
 import { ScheduleService } from '../schedule.service';
 import { DateTimeFormatService } from '../../../core/date-time-format/date-time-format.service';
+import { parseDbDateStr } from '../../../util/parse-db-date-str';
 
 describe('ScheduleMonthComponent', () => {
   let component: ScheduleMonthComponent;
@@ -120,7 +121,7 @@ describe('ScheduleMonthComponent', () => {
 
       // Assert
       // Middle index = floor(35/2) = 17
-      const middleDay = new Date(days[17]);
+      const middleDay = parseDbDateStr(days[17]);
       expect(result.getFullYear()).toBe(middleDay.getFullYear());
       expect(result.getMonth()).toBe(middleDay.getMonth());
       expect(result.getDate()).toBe(middleDay.getDate());
@@ -143,7 +144,7 @@ describe('ScheduleMonthComponent', () => {
 
       // Assert
       // Middle index = floor(42/2) = 21
-      const middleDay = new Date(days[21]);
+      const middleDay = parseDbDateStr(days[21]);
       expect(result.getFullYear()).toBe(middleDay.getFullYear());
       expect(result.getMonth()).toBe(middleDay.getMonth());
     });
@@ -190,7 +191,7 @@ describe('ScheduleMonthComponent', () => {
     it('should handle "other-month" class for padding days', () => {
       // Arrange
       mockScheduleService.getDayClass.and.callFake((day: string, ref?: Date) => {
-        const dayDate = new Date(day);
+        const dayDate = parseDbDateStr(day);
         if (ref && dayDate.getMonth() !== ref.getMonth()) {
           return 'other-month';
         }

src/app/features/schedule/schedule-month/schedule-month.component.ts

@@ -12,6 +12,7 @@ import { T } from '../../../t.const';
 import { ScheduleService } from '../schedule.service';
 import { LocaleDatePipe } from 'src/app/ui/pipes/locale-date.pipe';
 import { DateTimeFormatService } from 'src/app/core/date-time-format/date-time-format.service';
+import { parseDbDateStr } from 'src/app/util/parse-db-date-str';
 
 @Component({
   selector: 'schedule-month',
@@ -59,7 +60,7 @@ export class ScheduleMonthComponent {
     // Use the middle day as reference (around day 14-15 of the month)
     // This ensures we get a day that's actually in the target month
     const middleIndex = Math.floor(days.length / 2);
-    return new Date(days[middleIndex]);
+    return parseDbDateStr(days[middleIndex]);
   });
 
   T: typeof T = T;

src/app/features/schedule/schedule-week/schedule-week.component.ts

@@ -28,6 +28,7 @@ import { DRAG_DELAY_FOR_TOUCH } from '../../../app.constants';
 import { MatTooltip } from '@angular/material/tooltip';
 import { DateTimeFormatService } from '../../../core/date-time-format/date-time-format.service';
 import { LocaleDatePipe } from 'src/app/ui/pipes/locale-date.pipe';
+import { parseDbDateStr } from '../../../util/parse-db-date-str';
 import { formatMonthDay } from '../../../util/format-month-day.util';
 import { ScheduleWeekDragService } from './schedule-week-drag.service';
 import { calculatePlaceholderForGridMove } from './schedule-week-placeholder.util';
@@ -331,7 +332,7 @@ export class ScheduleWeekComponent implements OnInit, AfterViewInit, OnDestroy {
     if (!dayStr) {
       return '';
     }
-    const date = new Date(dayStr);
+    const date = parseDbDateStr(dayStr);
     if (Number.isNaN(date.getTime())) {
       return dayStr;
     }

src/app/features/schedule/schedule.service.ts

@@ -23,6 +23,7 @@ import { CalendarIntegrationService } from '../calendar-integration/calendar-int
 import { toSignal } from '@angular/core/rxjs-interop';
 import { TaskService } from '../tasks/task.service';
 import { startWith } from 'rxjs/operators';
+import { parseDbDateStr } from '../../util/parse-db-date-str';
 
 @Injectable({
   providedIn: 'root',
@@ -272,7 +273,7 @@ export class ScheduleService {
   }
 
   getDayClass(day: string, referenceMonth?: Date): string {
-    const dayDate = new Date(day);
+    const dayDate = parseDbDateStr(day);
     const today = new Date();
 
     // If referenceMonth is provided, use it to determine "current month"

src/app/util/parse-db-date-str.ts

@@ -0,0 +1,14 @@
+// Parse 'YYYY-MM-DD' string as a local date (not UTC)
+//
+// ⚠️ Important: new Date('2026-01-12') parses as UTC midnight, which becomes
+// the previous day when converted to local timezone (e.g., Europe/Berlin UTC+1).
+// This function parses the string as a local date to avoid timezone issues.
+//
+// Examples:
+// - new Date('2026-01-12') in Europe/Berlin → 2026-01-11 23:00 (UTC midnight)
+// - parseDbDateStr('2026-01-12') → 2026-01-12 00:00 (local midnight)
+
+export const parseDbDateStr = (dateStr: string): Date => {
+  const [year, month, day] = dateStr.split('-').map(Number);
+  return new Date(year, month - 1, day);
+};