fix(automations): step schedule scans by calendar day so DST changes don't skip dates (#19086)

The RRULE day scan stepped local midnight by a fixed 24h. Walking back
over a 23-hour spring-forward day landed at 23:00 two dates earlier, so
the latest-occurrence lookup used for catch-up runs skipped a date. It
returned a run a day too old for daily schedules, and a week too old for
weekly ones.

Step with setDate via a shared addLocalDays helper, in both the RRULE
scan and the cron feasibility scan. Add a regression test for
America/New_York and Australia/Lord_Howe. The test is skipped on Windows,
where unstubbing TZ doesn't restore the OS zone.

Co-authored-by: liutian <liutian.6000@bytedance.com>
Co-authored-by: Neil <4138956+nwparker@users.noreply.github.com>

src/shared/automation-cron-occurrence.ts

@@ -1,6 +1,5 @@
 import type { ParsedCron } from './automation-schedule-parsing'
 
-const DAY_MS = 24 * 60 * 60 * 1000
 const CRON_SCAN_DAYS = 9 * 366
 
 export function startOfLocalDay(timestamp: number): number {
@@ -9,6 +8,14 @@ export function startOfLocalDay(timestamp: number): number {
   return date.getTime()
 }
 
+// Why: calendar steps, not 24h ones; DST days are not 24 hours long.
+export function addLocalDays(day: number, days: number): number {
+  const date = new Date(day)
+  date.setDate(date.getDate() + days)
+  date.setHours(0, 0, 0, 0)
+  return date.getTime()
+}
+
 export function floorToMinute(timestamp: number): number {
   const date = new Date(timestamp)
   date.setSeconds(0, 0)
@@ -43,7 +50,7 @@ export function cronHasPossibleOccurrence(rule: ParsedCron, anchor: number): boo
     if (cronDateMatches(rule, day)) {
       return true
     }
-    day += DAY_MS
+    day = addLocalDays(day, 1)
   }
   return false
 }

src/shared/automation-schedule-dst.test.ts

@@ -0,0 +1,41 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { latestAutomationOccurrenceAtOrBefore } from './automation-schedule-occurrences'
+
+afterEach(() => vi.unstubAllEnvs())
+
+// Why: on Windows, unsetting a stubbed TZ doesn't restore the OS zone, so it would leak into later tests.
+describe.skipIf(process.platform === 'win32')('automation calendar days across DST', () => {
+  it.each([
+    {
+      timezone: 'America/New_York',
+      now: '2026-03-09T00:30:00-04:00',
+      expected: '2026-03-08T23:30:00-04:00'
+    },
+    {
+      timezone: 'Australia/Lord_Howe',
+      now: '2026-10-05T00:15:00+11:00',
+      expected: '2026-10-04T23:30:00+11:00'
+    }
+  ])(
+    'finds the previous calendar day after spring-forward in $timezone',
+    ({ timezone, now, expected }) => {
+      vi.stubEnv('TZ', timezone)
+      // Precondition: the stub took and the offset changed in the preceding two days.
+      expect(new Date(now).getTimezoneOffset()).not.toBe(
+        new Date(Date.parse(now) - 2 * 24 * 60 * 60 * 1000).getTimezoneOffset()
+      )
+      for (const rrule of [
+        'FREQ=DAILY;BYHOUR=23;BYMINUTE=30',
+        'FREQ=WEEKLY;BYDAY=SU;BYHOUR=23;BYMINUTE=30'
+      ]) {
+        expect(
+          latestAutomationOccurrenceAtOrBefore(
+            rrule,
+            Date.parse('2026-01-01T00:00:00Z'),
+            Date.parse(now)
+          )
+        ).toBe(Date.parse(expected))
+      }
+    }
+  )
+})

src/shared/automation-schedule-occurrences.ts

@@ -1,8 +1,12 @@
 import type { AutomationSchedulePreset } from './automations-types'
 import { parseSchedule, type ParsedRrule } from './automation-schedule-parsing'
-import { cronMatches, floorToMinute, startOfLocalDay } from './automation-cron-occurrence'
+import {
+  addLocalDays,
+  cronMatches,
+  floorToMinute,
+  startOfLocalDay
+} from './automation-cron-occurrence'
 
-const DAY_MS = 24 * 60 * 60 * 1000
 const HOUR_MS = 60 * 60 * 1000
 const MINUTE_MS = 60 * 1000
 // Why: valid cron expressions like Feb 29 can have an 8-year gap across non-leap centuries.
@@ -35,7 +39,7 @@ function scanDayCandidates(rule: ParsedRrule, anchor: number, direction: 1 | -1)
         return candidate
       }
     }
-    day += direction * DAY_MS
+    day = addLocalDays(day, direction)
   }
   return null
 }