fix(core): preserve calendar anchors after DST gaps

packages/core/src/__tests__/scheduled-task.test.ts

@@ -18,6 +18,7 @@
  */
 
 import assert from 'node:assert/strict';
+import { execFileSync } from 'node:child_process';
 import { describe, it } from 'node:test';
 import { markPersisted } from '../persisted-value.js';
 import {
@@ -181,6 +182,95 @@ describe('scheduled-task catalog', () => {
     assert.equal(new Date(february!).getDate(), 28);
   });
 
+  it('reconstructs daily and weekly wall times after a DST gap', () => {
+    const moduleUrl = new URL('../scheduled-task.js', import.meta.url).href;
+    const cases = [
+      {
+        timezone: 'America/New_York',
+        anchor: '2026-03-01T02:30:15.125-05:00',
+        after: '2026-03-08T04:00:00-04:00',
+        daily: '2026-03-09T02:30:15.125-04:00',
+        weekly: '2026-03-15T02:30:15.125-04:00',
+      },
+      {
+        timezone: 'America/New_York',
+        anchor: '2026-03-03T02:30:15.125-05:00',
+        after: '2026-03-08T00:00:00-05:00',
+        daily: '2026-03-08T03:30:15.125-04:00',
+        weekly: '2026-03-10T02:30:15.125-04:00',
+      },
+      {
+        timezone: 'Australia/Lord_Howe',
+        anchor: '2026-09-27T02:15:15.125+10:30',
+        after: '2026-10-04T04:00:00+11:00',
+        daily: '2026-10-05T02:15:15.125+11:00',
+        weekly: '2026-10-11T02:15:15.125+11:00',
+      },
+      {
+        timezone: 'America/New_York',
+        anchor: '2026-10-25T01:30:15.125-04:00',
+        after: '2026-11-01T01:30:15.125-05:00',
+        daily: '2026-11-02T01:30:15.125-05:00',
+        weekly: '2026-11-08T01:30:15.125-05:00',
+      },
+    ];
+    for (const scenario of cases) {
+      const script = `
+        import { computeNextFireAt } from ${JSON.stringify(moduleUrl)};
+        const scenario = ${JSON.stringify(scenario)};
+        const result = ['daily', 'weekly'].map((recurrence) =>
+          computeNextFireAt({
+            kind: 'calendar', recurrence, anchorAt: Date.parse(scenario.anchor),
+          }, Date.parse(scenario.after)));
+        process.stdout.write(JSON.stringify(result));
+      `;
+      const actual = JSON.parse(
+        execFileSync(process.execPath, ['--input-type=module', '-e', script], {
+          env: { ...process.env, TZ: scenario.timezone },
+          encoding: 'utf8',
+        }),
+      );
+      assert.deepEqual(
+        actual,
+        [Date.parse(scenario.daily), Date.parse(scenario.weekly)],
+        `${scenario.timezone}: ${scenario.after}`,
+      );
+    }
+  });
+
+  it('returns to the anchor wall time after firing on a DST gap day', () => {
+    const moduleUrl = new URL('../scheduled-task.js', import.meta.url).href;
+    const script = `
+      import { computeNextFireAt } from ${JSON.stringify(moduleUrl)};
+      const anchors = {
+        daily: '2026-03-01T02:30:15.125-05:00',
+        weekly: '2026-03-01T02:30:15.125-05:00',
+        monthly: '2026-01-08T02:30:15.125-05:00',
+      };
+      const result = Object.entries(anchors).map(([recurrence, anchor]) => {
+        const schedule = { kind: 'calendar', recurrence, anchorAt: Date.parse(anchor) };
+        let after = Date.parse('2026-03-08T00:00:00-05:00');
+        return Array.from({length: 2}, () => {
+          after = computeNextFireAt(schedule, after);
+          return after;
+        });
+      });
+      process.stdout.write(JSON.stringify(result));
+    `;
+    const actual = JSON.parse(
+      execFileSync(process.execPath, ['--input-type=module', '-e', script], {
+        env: { ...process.env, TZ: 'America/New_York' },
+        encoding: 'utf8',
+      }),
+    );
+    const gap = Date.parse('2026-03-08T03:30:15.125-04:00');
+    assert.deepEqual(actual, [
+      [gap, Date.parse('2026-03-09T02:30:15.125-04:00')],
+      [gap, Date.parse('2026-03-15T02:30:15.125-04:00')],
+      [gap, Date.parse('2026-04-08T02:30:15.125-04:00')],
+    ]);
+  });
+
   it('rejects tasks whose first fire is not before expiration', () => {
     const now = Date.UTC(2026, 0, 5, 8, 0, 0);
     const result = normalizeCreateScheduledTaskInput(

packages/core/src/scheduled-task.ts

@@ -616,34 +616,35 @@ function asFiniteNumber(value: unknown): number | null {
   return null;
 }
 
+/**
+ * Calendar recurrence uses Date's local-time rules: shift gaps forward and take
+ * the first repeated time. Rebuild from the anchor so gap shifts do not carry
+ * into later cycles; cron instead skips nonexistent local times.
+ */
 function nextCalendarFireAt(
   schedule: Extract<ScheduledTaskSchedule, { kind: 'calendar' }>,
   after: number,
 ): number {
   const anchor = new Date(schedule.anchorAt);
-  if (schedule.recurrence === 'daily') {
-    const next = new Date(after);
-    next.setHours(
-      anchor.getHours(),
-      anchor.getMinutes(),
-      anchor.getSeconds(),
-      anchor.getMilliseconds(),
-    );
-    if (next.getTime() <= after) next.setDate(next.getDate() + 1);
-    return next.getTime();
-  }
-  if (schedule.recurrence === 'weekly') {
-    const next = new Date(after);
-    next.setHours(
-      anchor.getHours(),
-      anchor.getMinutes(),
-      anchor.getSeconds(),
-      anchor.getMilliseconds(),
-    );
-    const dayDelta = (anchor.getDay() - next.getDay() + 7) % 7;
-    next.setDate(next.getDate() + dayDelta);
-    if (next.getTime() <= after) next.setDate(next.getDate() + 7);
-    return next.getTime();
+  if (schedule.recurrence === 'daily' || schedule.recurrence === 'weekly') {
+    const base = new Date(after);
+    const dayOffset =
+      schedule.recurrence === 'daily' ? 0 : (anchor.getDay() - base.getDay() + 7) % 7;
+    const periodDays = schedule.recurrence === 'daily' ? 1 : 7;
+    // Build each occurrence from the anchor: a DST gap may normalize today's
+    // wall time, but that adjusted hour must not carry into another date.
+    const occurrence = (offset: number): number =>
+      new Date(
+        base.getFullYear(),
+        base.getMonth(),
+        base.getDate() + offset,
+        anchor.getHours(),
+        anchor.getMinutes(),
+        anchor.getSeconds(),
+        anchor.getMilliseconds(),
+      ).getTime();
+    const next = occurrence(dayOffset);
+    return next > after ? next : occurrence(dayOffset + periodDays);
   }
   const afterDate = new Date(after);
   for (let offset = 0; offset <= 480; offset += 1) {