fix(darwin): correct timezone offset across month/year boundaries

src/__tests__/schedulers/darwin.test.ts

@@ -1,4 +1,4 @@
-import { describe, it, expect } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
 import {
   generatePlist,
   cronToCalendarInterval,
@@ -197,6 +197,22 @@ describe('computeTimezoneOffsetMinutes', () => {
     expect(tokyoAheadOfNY).toBeGreaterThanOrEqual(13 * 60);
     expect(tokyoAheadOfNY).toBeLessThanOrEqual(14 * 60);
   });
+
+  it('stays correct when the instant straddles a month boundary (regression)', () => {
+    // At 2026-07-31T23:30Z it is still Jul 31 in UTC/NY but already Aug 1 in Tokyo,
+    // so a day-of-month-only calc jumps 31 -> 1 and injects a ~30-day error.
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2026-07-31T23:30:00Z'));
+    try {
+      const toTokyo = computeTimezoneOffsetMinutes('Asia/Tokyo');
+      const toNY = computeTimezoneOffsetMinutes('America/New_York');
+      const tokyoAheadOfNY = toNY - toTokyo;
+      expect(tokyoAheadOfNY).toBeGreaterThanOrEqual(13 * 60);
+      expect(tokyoAheadOfNY).toBeLessThanOrEqual(14 * 60);
+    } finally {
+      vi.useRealTimers();
+    }
+  });
 });
 
 describe('adjustCalendarIntervalsForTimezone', () => {

src/schedulers/darwin.ts

@@ -46,15 +46,17 @@ export function computeTimezoneOffsetMinutes(targetTz: string): number {
     ...(tz ? { timeZone: tz } : {}),
   });
 
-  const toMinutesSinceEpochDay = (parts: Intl.DateTimeFormatPart[]) => {
-    const day = parseInt(parts.find(p => p.type === 'day')!.value, 10);
-    const h = parseInt(parts.find(p => p.type === 'hour')!.value, 10);
-    const m = parseInt(parts.find(p => p.type === 'minute')!.value, 10);
-    return day * 1440 + h * 60 + m;
+  const toWallMinutes = (parts: Intl.DateTimeFormatPart[]) => {
+    const get = (type: string) => parseInt(parts.find(p => p.type === type)!.value, 10);
+    // Use the full year/month/day so the subtraction stays correct across month
+    // and year boundaries (day-of-month alone jumps e.g. 31 -> 1, a ~30-day error).
+    return Math.round(
+      Date.UTC(get('year'), get('month') - 1, get('day'), get('hour'), get('minute')) / 60000,
+    );
   };
 
-  const localMinutes = toMinutesSinceEpochDay(fmt().formatToParts(now));
-  const targetMinutes = toMinutesSinceEpochDay(fmt(targetTz).formatToParts(now));
+  const localMinutes = toWallMinutes(fmt().formatToParts(now));
+  const targetMinutes = toWallMinutes(fmt(targetTz).formatToParts(now));
 
   return localMinutes - targetMinutes;
 }