fix(due,store,ui): month grid steps by calendar day so dst cannot repeat one

src/lib/due.ts

@@ -29,6 +29,16 @@ export function shiftDays(key: string, days: number): string {
   return localKey(date);
 }
 
+export function monthGrid(month: Date): Date[] {
+  const year = month.getFullYear();
+  const index = month.getMonth();
+  const offset = (new Date(year, index, 1).getDay() + 6) % 7;
+  return Array.from(
+    { length: 42 },
+    (_, cell) => new Date(year, index, 1 - offset + cell),
+  );
+}
+
 export function daysFromToday(key: string, from = today()): number {
   return Math.round((fromKey(key).getTime() - fromKey(from).getTime()) / 86_400_000);
 }

src/ui/day-grid.svelte

@@ -5,7 +5,7 @@
 <script lang="ts">
   import ChevronLeft from "@lucide/svelte/icons/chevron-left";
   import ChevronRight from "@lucide/svelte/icons/chevron-right";
-  import { fromKey, localKey } from "../lib/due";
+  import { fromKey, localKey, monthGrid } from "../lib/due";
   import { WEEKDAYS } from "../lib/vocab";
   import { cn } from "../lib/utils";
 
@@ -20,7 +20,6 @@
 
   let { anchor, tone, onpick, onhover, today }: Props = $props();
 
-  const DAY = 86_400_000;
   const monthName = new Intl.DateTimeFormat("en-US", { month: "long" });
   const todayKey = $derived(today ?? localKey(new Date()));
 
@@ -32,20 +31,13 @@
     return new Date(base.getFullYear(), base.getMonth(), 1);
   });
 
-  const days = $derived.by(() => {
-    const offset = (month.getDay() + 6) % 7;
-    const start = new Date(month.getTime() - offset * DAY);
-    start.setHours(0, 0, 0, 0);
-    return Array.from({ length: 42 }, (_, index) => {
-      const date = new Date(start.getTime() + index * DAY);
-      date.setHours(0, 0, 0, 0);
-      return {
+  const days = $derived(
+    monthGrid(month).map((date) => ({
       date,
       inMonth: date.getMonth() === month.getMonth(),
       key: localKey(date),
-      };
-    });
-  });
+    })),
+  );
 
   function shiftMonth(delta: number) {
     picked = new Date(month.getFullYear(), month.getMonth() + delta, 1);

src/vault/store.svelte.ts

@@ -4,7 +4,7 @@ import { isArchivePath, toBoardPath } from "../model/archive";
 import { parseBoard, sphereOf } from "../model/board";
 import { anchorDate, isOpen, STATUS_ORDER } from "../model/types";
 import type { Board, Status, Task } from "../model/types";
-import { daysFromToday, fromKey, localKey, shiftDays } from "../lib/due";
+import { daysFromToday, fromKey, localKey, monthGrid, shiftDays } from "../lib/due";
 import { laneKey, parseLaneKey } from "../lib/keys";
 import type {
   BeaverCalendarSettings,
@@ -13,7 +13,6 @@ import type {
   ViewMode,
 } from "../settings";
 
-const DAY = 86_400_000;
 
 export interface Group {
   key: string;
@@ -449,13 +448,9 @@ export class BoardStore {
   monthDays = $derived.by<DaySlot[]>(() => {
     const anchor = fromKey(this.monthKey);
     const first = new Date(anchor.getFullYear(), anchor.getMonth(), 1);
-    const offset = (first.getDay() + 6) % 7;
-    const start = new Date(first.getTime() - offset * DAY);
     const todayKey = this.today;
     const byDay = this.byDay;
-    return Array.from({ length: 42 }, (_, index) => {
-      const date = new Date(start.getTime() + index * DAY);
-      date.setHours(0, 0, 0, 0);
+    return monthGrid(first).map((date) => {
       const key = localKey(date);
       return {
         date,

tests/due.test.ts

@@ -1,8 +1,10 @@
-import { describe, expect, it } from "vitest";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
 import {
   daysFromToday,
   dueLabel,
   dueTone,
+  localKey,
+  monthGrid,
   nextFriday,
 } from "../src/lib/due";
 
@@ -26,3 +28,30 @@ describe("due helpers read the day they are given", () => {
     expect(nextFriday("2026-08-28")).toBe("2026-09-04");
   });
 });
+
+describe("monthGrid steps by calendar day", () => {
+  const zone = process.env.TZ;
+  beforeAll(() => {
+    process.env.TZ = "Europe/Berlin";
+  });
+  afterAll(() => {
+    process.env.TZ = zone;
+  });
+
+  it("keeps every day once across the autumn clock change", () => {
+    const keys = monthGrid(new Date(2026, 9, 1)).map(localKey);
+    expect(keys).toHaveLength(42);
+    expect(new Set(keys).size).toBe(42);
+    expect(keys[0]).toBe("2026-09-28");
+    expect(keys[27]).toBe("2026-10-25");
+    expect(keys[28]).toBe("2026-10-26");
+    expect(keys[41]).toBe("2026-11-08");
+  });
+
+  it("keeps every day once across the spring clock change", () => {
+    const keys = monthGrid(new Date(2027, 2, 1)).map(localKey);
+    expect(new Set(keys).size).toBe(42);
+    expect(keys).toContain("2027-03-28");
+    expect(keys).toContain("2027-03-29");
+  });
+});