From 080afa9cffdf7b92201026ff279392cbead4efc9 Mon Sep 17 00:00:00 2001
From: Maurice <mauriceboe@icloud.com>
Date: Fri, 9 Oct 2026 10:11:36 +0200
Subject: [PATCH] fix(trips): keep the trip length across a DST change when the
 phone moves the start

The phone sheet added the old length with a local setDate on a UTC midnight date, so in zones east of UTC a range crossing the start of summer time came out one day short. It now counts UTC calendar days like the desktop dialog.
---
 client/src/components/Trips/tripFormModel.test.ts | 14 +++++++++++++-
 client/src/components/Trips/tripFormModel.ts      | 11 ++++-------
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/client/src/components/Trips/tripFormModel.test.ts b/client/src/components/Trips/tripFormModel.test.ts
index c21e1048c..5b1a44e85 100644
--- a/client/src/components/Trips/tripFormModel.test.ts
+++ b/client/src/components/Trips/tripFormModel.test.ts
@@ -1,4 +1,4 @@
-// FE-COMP-TRIPFORMMODEL-001 to -006: the trip form checks the dialog and the phone sheet share.
+// FE-COMP-TRIPFORMMODEL-001 to -007: the trip form checks the dialog and the phone sheet share.
 import { MAX_TRIP_DAYS, addIsoDays } from '@trek/shared';
 
 import { endDateForNewStart, tripFormError } from './tripFormModel';
@@ -52,4 +52,16 @@ describe('endDateForNewStart', () => {
     expect(endDateForNewStart('2026-05-08', '2026-05-03', '2026-05-05')).toBe('2026-05-05');
     expect(endDateForNewStart('2026-05-01', '2026-05-08', '')).toBe('2026-05-08');
   });
+
+  it('FE-COMP-TRIPFORMMODEL-007: the kept length counts calendar days across a DST change', () => {
+    const runnerTimeZone = process.env.TZ;
+    process.env.TZ = 'Europe/Berlin';
+    try {
+      // Summer time starts on 2026-03-29 in Berlin.
+      expect(endDateForNewStart('2026-03-20', '2026-03-25', '2026-03-27')).toBe('2026-04-01');
+    } finally {
+      if (runnerTimeZone === undefined) delete process.env.TZ;
+      else process.env.TZ = runnerTimeZone;
+    }
+  });
 });
diff --git a/client/src/components/Trips/tripFormModel.ts b/client/src/components/Trips/tripFormModel.ts
index a8db48e84..a1d38bf42 100644
--- a/client/src/components/Trips/tripFormModel.ts
+++ b/client/src/components/Trips/tripFormModel.ts
@@ -1,4 +1,4 @@
-import { MAX_TRIP_DAYS, tripSpanDays } from '@trek/shared';
+import { MAX_TRIP_DAYS, addIsoDays, tripSpanDays } from '@trek/shared';
 
 type T = (key: string, params?: Record<string, string | number>) => string;
 
@@ -34,12 +34,9 @@ export function tripFormError(
  */
 export function endDateForNewStart(startDate: string, endDate: string, value: string): string {
   if (value && endDate && startDate && endDate >= startDate) {
-    const duration = Math.round(
-      (new Date(endDate + 'T00:00:00Z').getTime() - new Date(startDate + 'T00:00:00Z').getTime()) / 86400000
-    );
-    const newEnd = new Date(value + 'T00:00:00Z');
-    newEnd.setDate(newEnd.getDate() + duration);
-    return newEnd.toISOString().split('T')[0];
+    // Counted in UTC calendar days: a local-time shift across a DST change used to
+    // drop or add a day.
+    return addIsoDays(value, tripSpanDays(startDate, endDate) - 1);
   }
   if (value && (!endDate || endDate < value)) return value;
   return endDate;