diff --git a/src/app/services/planning-calculation.service.spec.ts b/src/app/services/planning-calculation.service.spec.ts
index ec88d8d..d52bf0a 100644
--- a/src/app/services/planning-calculation.service.spec.ts
+++ b/src/app/services/planning-calculation.service.spec.ts
@@ -1,7 +1,7 @@
 import { provideZonelessChangeDetection } from '@angular/core';
 import { TestBed } from '@angular/core/testing';
 import { vi } from '../../testing/vitest-compat';
-import { CalendarService } from './calendar-service';
+import { CalendarService, SelectableDayType } from './calendar-service';
 import { CalendarSettingsService } from './calendar-settings-service';
 import type {
 	PlanOutcome,
@@ -11,6 +11,7 @@ import type {
 	PlanningReason,
 } from './leave-planner.service';
 import { PlanningCalculationService } from './planning-calculation.service';
+import { RuleService, RuleType } from './rule-service';
 import { UserInputService } from './user-input-service';
 
 function deferred<T>() {
@@ -119,6 +120,33 @@ describe('PlanningCalculationService', () => {
 		expect(submission.labels).toBeDefined();
 	});
 
+	it('hands TIME_WINDOW rules the picked calendar dates east of Paris', () => {
+		const previousTimeZone = process.env['TZ'];
+		process.env['TZ'] = 'Pacific/Auckland';
+		try {
+			TestBed.inject(RuleService).addRule({
+				id: 'christmas',
+				name: 'Christmas',
+				type: RuleType.TIME_WINDOW,
+				enabled: true,
+				force: 'obligation',
+				startDate: new Date(2026, 11, 20),
+				endDate: new Date(2026, 11, 31),
+				placeDays: 3,
+				placeType: SelectableDayType.CP,
+			});
+			queue(fakeCalculation('first'));
+			service.start();
+			const submitted = create.mock.calls[0][0].rules.find(
+				(rule: { id: string }) => rule.id === 'christmas',
+			);
+			expect(submitted).toMatchObject({ startDate: '2026-12-20', endDate: '2026-12-31' });
+		} finally {
+			// bun test defaults to UTC without setting TZ; deleting it would fall back to the host zone.
+			process.env['TZ'] = previousTimeZone ?? 'UTC';
+		}
+	});
+
 	it('reports the stage and submission of the latest calculation only, until Cancel', () => {
 		queue(fakeCalculation('old'), fakeCalculation('latest'));
 		service.start();
diff --git a/src/app/services/planning-calculation.service.ts b/src/app/services/planning-calculation.service.ts
index 84a615d..b66d7fc 100644
--- a/src/app/services/planning-calculation.service.ts
+++ b/src/app/services/planning-calculation.service.ts
@@ -11,7 +11,7 @@ import type {
 	PlanningRequest,
 	PlanningSnapshot,
 } from './leave-planner.service';
-import { RuleService } from './rule-service';
+import { RuleService, RuleType } from './rule-service';
 import { TranslationService } from './translation-service';
 import { UserInputService } from './user-input-service';
 
@@ -85,7 +85,15 @@ export class PlanningCalculationService {
 					: undefined,
 				expiryDate: dayjs(balance.expiryDate).format('YYYY-MM-DD'),
 			})),
-			rules: this.rules.rules,
+			rules: this.rules.rules.map((rule) =>
+				rule.type === RuleType.TIME_WINDOW
+					? {
+							...rule,
+							startDate: dayjs(rule.startDate).format('YYYY-MM-DD'),
+							endDate: dayjs(rule.endDate).format('YYYY-MM-DD'),
+						}
+					: rule,
+			),
 		};
 	});