fix(trips): count trip dates in calendar days, the same on REST and MCP

Filling in a missing end date and keeping a trip's length when its start
moved both shifted a local-time Date, so a trip crossing a DST change
lost or gained a day. Both now use addIsoDays. The one-date-makes-a-week
default moves from the REST controller into TripsService.create, so the
MCP create_trip tool no longer stores a half-dated trip, and the trip and
its days are written in one transaction.

client/src/components/Trips/TripFormModal.test.tsx

@@ -442,6 +442,20 @@ describe('TripFormModal', () => {
     }));
   });
 
+  it('FE-COMP-TRIPFORM-038: moving the start across a DST change keeps the trip length in calendar days', async () => {
+    const user = userEvent.setup();
+    const onSave = vi.fn().mockResolvedValue({});
+    const trip = buildTrip({ id: 1, title: 'Spring Trip', start_date: '2026-03-20', end_date: '2026-03-29' });
+    render(<TripFormModal {...defaultProps} trip={trip} onSave={onSave} />);
+
+    await changeStartDate(user, '2026-03-25');
+    await user.click(screen.getByRole('button', { name: /Update/i }));
+    await user.click(await screen.findByRole('button', { name: /Update/i }));
+
+    await waitFor(() => expect(onSave).toHaveBeenCalled());
+    expect(onSave).toHaveBeenCalledWith(expect.objectContaining({ start_date: '2026-03-25', end_date: '2026-04-03' }));
+  });
+
   it('FE-COMP-TRIPFORM-036: picking "Shift everything" sends shift_all', async () => {
     const user = userEvent.setup();
     const onSave = vi.fn().mockResolvedValue({});

client/src/components/Trips/TripFormModal.tsx

@@ -11,7 +11,7 @@ import { useTranslation } from '../../i18n'
 import { CustomDatePicker } from '../shared/CustomDateTimePicker'
 import { normalizeImageFile } from '../../utils/convertHeic'
 import { getApiErrorMessage, type Trip } from '../../types'
-import { MAX_TRIP_DAYS, tripSpanDays, type TripCreateRequest } from '@trek/shared'
+import { MAX_TRIP_DAYS, addIsoDays, tripSpanDays, type TripCreateRequest } from '@trek/shared'
 import { NumericInput } from '../shared/NumericInput'
 import { currenciesWith, SYMBOLS } from '../Budget/BudgetPanel.constants'
 import TripDateReview, { dateReviewIsWide } from './TripDateReview'
@@ -373,12 +373,9 @@ export default function TripFormModal({ isOpen, onClose, onSave, trip, onCoverUp
       if (!prev.end_date || prev.end_date < value) {
         next.end_date = value
       } else if (prev.start_date) {
-        const oldStart = new Date(prev.start_date + 'T00:00:00Z')
-        const oldEnd = new Date(prev.end_date + 'T00:00:00Z')
-        const duration = Math.round((oldEnd.getTime() - oldStart.getTime()) / 86400000)
-        const newEnd = new Date(value + 'T00:00:00Z')
-        newEnd.setDate(newEnd.getDate() + duration)
-        next.end_date = newEnd.toISOString().split('T')[0]
+        // Keep the trip's length, counted in UTC calendar days: a local-time
+        // shift across a DST change used to drop or add a day.
+        next.end_date = addIsoDays(value, tripSpanDays(prev.start_date, prev.end_date) - 1)
       }
     }
     return next

server/src/nest/trips/trips.controller.ts

@@ -50,9 +50,6 @@ export const TRIP_COVER_FILE_FILTER: Options['fileFilter'] = (_req, file, cb) =>
   else cb(new Error('Only jpg, png, gif, webp images allowed'));
 };
 
-const toDateStr = (d: Date) => d.toISOString().slice(0, 10);
-const addDays = (d: Date, n: number) => { const r = new Date(d); r.setDate(r.getDate() + n); return r; };
-
 /**
  * /api/trips — the trip aggregate root.
  *
@@ -115,10 +112,8 @@ export class TripsController {
     }
     // Presence/shape validation happens in the ZodValidationPipe (tripCreateRequestSchema).
     const { title, description, currency, reminder_days, day_count } = body;
-    let start_date: string | null = body.start_date || null;
-    let end_date: string | null = body.end_date || null;
-    if (start_date && !end_date) end_date = toDateStr(addDays(new Date(start_date), 6));
-    else if (!start_date && end_date) start_date = toDateStr(addDays(new Date(end_date), -6));
+    const start_date: string | null = body.start_date || null;
+    const end_date: string | null = body.end_date || null;
     if (start_date && end_date && new Date(end_date) < new Date(start_date)) {
       throw new HttpException({ error: 'End date must be after start date' }, 400);
     }

server/src/nest/trips/trips.service.ts

@@ -28,6 +28,7 @@ import { TourWaypoints } from '../../db/entities/TourWaypoints.entity';
 import type { TourWaypointRow } from '../../db/repositories/TourWaypoints.repository';
 import {
   MAX_TRIP_DAYS,
+  addIsoDays,
   planDayGrid,
   resolveDayGridRange,
   tripSpanDays,
@@ -405,23 +406,34 @@ export class TripsService {
   }
 
   async create(userId: number, data: CreateTripData) {
-    if (data.start_date && data.end_date) assertTripSpan(data.start_date, data.end_date);
+    // One given date makes a week, on REST and MCP alike. Calendar arithmetic
+    // in UTC: a local-time Date shifted across a DST change lost or gained a day.
+    let startDate = data.start_date || null;
+    let endDate = data.end_date || null;
+    if (startDate && !endDate) endDate = addIsoDays(startDate, 6);
+    else if (!startDate && endDate) startDate = addIsoDays(endDate, -6);
+    if (startDate && endDate) assertTripSpan(startDate, endDate);
     const rd = data.reminder_days !== undefined
       ? (Number(data.reminder_days) >= 0 && Number(data.reminder_days) <= 30 ? Number(data.reminder_days) : 3)
       : 3;
+    const currency = data.currency || (await this.defaultCurrencyFor(userId));
 
-    const tripId = await this.tripsRepo.insertTrip({ // TP18
-      user_id: userId,
-      title: data.title,
-      description: data.description || null,
-      start_date: data.start_date || null,
-      end_date: data.end_date || null,
-      currency: data.currency || (await this.defaultCurrencyFor(userId)),
-      reminder_days: rd,
+    // The trip and its days commit together: a failure in between used to leave
+    // a dated trip without a single day.
+    const tripId = await this.uow.transactional(async () => {
+      const id = await this.tripsRepo.insertTrip({ // TP18
+        user_id: userId,
+        title: data.title,
+        description: data.description || null,
+        start_date: startDate,
+        end_date: endDate,
+        currency,
+        reminder_days: rd,
+      });
+      await this.generateDays(id, startDate, endDate, data.day_count);
+      return id;
     });
 
-    await this.generateDays(tripId, data.start_date || null, data.end_date || null, data.day_count);
-
     const trip = await this.tripsRepo.findForViewer(tripId, userId); // TP19 — the creator always owns it, so the access predicate is trivially satisfied
     return { trip, tripId, reminderDays: rd };
   }

server/tests/unit/nest/trips.controller.test.ts

@@ -161,10 +161,10 @@ describe('TripsController (parity with the legacy /api/trips route)', async () =
       expect(tripCreateRequestSchema.safeParse({ title: '' }).success).toBe(false);
     });
 
-    it('infers end_date from start_date (+6 days) and creates', async () => {
+    it('passes a single start date on as it is; the service fills in the week', async () => {
       const create = vi.fn().mockReturnValue({ trip: { id: 9 }, tripId: 9, reminderDays: 0 });
       await tc(svc({ create } as Partial<TripsService>)).create(user, { title: 'T', start_date: '2026-07-01' }, req);
-      expect(create).toHaveBeenCalledWith(1, expect.objectContaining({ start_date: '2026-07-01', end_date: '2026-07-07' }));
+      expect(create).toHaveBeenCalledWith(1, expect.objectContaining({ start_date: '2026-07-01', end_date: null }));
     });
 
     it('400 when end_date precedes start_date', async () => {
@@ -173,10 +173,10 @@ describe('TripsController (parity with the legacy /api/trips route)', async () =
       });
     });
 
-    it('infers start_date from end_date (-6 days) and parses day_count', async () => {
+    it('passes a single end date on as it is and parses day_count', async () => {
       const create = vi.fn().mockReturnValue({ trip: { id: 9 }, tripId: 9, reminderDays: 0 });
       await tc(svc({ create } as Partial<TripsService>)).create(user, prePipeCreateBody({ title: 'T', end_date: '2026-07-07', day_count: '40' }), req);
-      expect(create).toHaveBeenCalledWith(1, expect.objectContaining({ start_date: '2026-07-01', end_date: '2026-07-07', day_count: 40 }));
+      expect(create).toHaveBeenCalledWith(1, expect.objectContaining({ start_date: null, end_date: '2026-07-07', day_count: 40 }));
     });
 
     it('clamps a non-numeric day_count to the default of 7', async () => {

server/tests/unit/nest/trips.service.test.ts

@@ -1116,6 +1116,24 @@ describe('folded trip CRUD', () => {
     expect(getDays(tripId)).toHaveLength(3);
   });
 
+  it('TRIP-SVC-090: one given date makes a week, counted in calendar days across a DST change', async () => {
+    const { user } = createUser(testDb);
+    const fromStart = await svc.create(user.id, { title: 'Spring', start_date: '2026-03-25' });
+    expect(fromStart.trip).toMatchObject({ start_date: '2026-03-25', end_date: '2026-03-31' });
+    expect(getDays(fromStart.tripId)).toHaveLength(7);
+    const fromEnd = await svc.create(user.id, { title: 'Autumn', end_date: '2026-10-30' });
+    expect(fromEnd.trip).toMatchObject({ start_date: '2026-10-24', end_date: '2026-10-30' });
+  });
+
+  it('TRIP-SVC-091: the trip and its days commit together', async () => {
+    const { user } = createUser(testDb);
+    const before = (testDb.prepare('SELECT COUNT(*) AS c FROM trips').get() as { c: number }).c;
+    const generate = vi.spyOn(svc, 'generateDays').mockRejectedValueOnce(new Error('disk I/O error'));
+    await expect(svc.create(user.id, { title: 'Half', start_date: '2026-05-01', end_date: '2026-05-03' })).rejects.toThrow('disk I/O error');
+    generate.mockRestore();
+    expect((testDb.prepare('SELECT COUNT(*) AS c FROM trips').get() as { c: number }).c).toBe(before);
+  });
+
   it('TRIP-SVC-080: create without a currency takes the display currency, admin default included, else EUR', async () => {
     const { user } = createUser(testDb);
     const setUser = (value: string) =>