fix(timezone): preserve ambiguous date semantics

Author: j15z

apps/sim/lib/core/utils/timezone.test.ts

@@ -136,11 +136,11 @@ describe('zonedWallClockToUtc', () => {
   )
 
   it.each([
-    ['America/New_York', '2026-11-01T01:30', '2026-11-01T05:30:00.000Z', '-04:00'],
-    ['Europe/Berlin', '2026-10-25T02:30', '2026-10-25T00:30:00.000Z', '+02:00'],
-    ['Australia/Lord_Howe', '2026-04-05T01:45', '2026-04-04T14:45:00.000Z', '+11:00'],
+    ['America/New_York', '2026-11-01T01:30', '2026-11-01T06:30:00.000Z', '-05:00'],
+    ['Europe/Berlin', '2026-10-25T02:30', '2026-10-25T01:30:00.000Z', '+01:00'],
+    ['Australia/Lord_Howe', '2026-04-05T01:45', '2026-04-04T15:15:00.000Z', '+10:30'],
   ])(
-    'keeps the earlier instant for an ambiguous fall-back wall-clock in %s',
+    'chooses the later post-transition instant for an ambiguous fall-back wall-clock in %s',
     (timeZone, wallClock, expectedInstant, expectedOffset) => {
       const instant = zonedWallClockToUtc(wallClock, timeZone)
       const stampedWallClock = zonedWallClockWithOffset(wallClock, timeZone)

apps/sim/lib/core/utils/timezone.ts

@@ -217,7 +217,7 @@ function resolveZonedWallClock(wallClock: string, timeZone: string): ZonedWallCl
   })
   const exactCandidate = candidates
     .filter(({ wallClockMs }) => wallClockMs === utcGuess)
-    .sort((a, b) => a.instantMs - b.instantMs)[0]
+    .sort((a, b) => b.instantMs - a.instantMs)[0]
   const compatibleCandidate = candidates
     .filter(({ wallClockMs }) => wallClockMs > utcGuess)
     .sort((a, b) => a.wallClockMs - b.wallClockMs || a.instantMs - b.instantMs)[0]
@@ -236,7 +236,7 @@ function resolveZonedWallClock(wallClock: string, timeZone: string): ZonedWallCl
  * date (including future ones whose offset differs from today's) and across DST:
  * a naive single pass reads the offset on the wrong side of a same-day boundary
  * — notably the autumn fall-back hour — and lands an hour off. For an ambiguous
- * fall-back wall-clock the earlier instant is chosen; a
+ * fall-back wall-clock the later, post-transition instant is chosen; a
  * wall-clock in the spring-forward gap (a nonexistent local hour) has no
  * self-consistent instant and resolves forward by the DST shift, matching how
  * calendar apps treat that once-a-year hour.

apps/sim/lib/table/dates.test.ts

@@ -97,8 +97,26 @@ describe('normalizeDateCellValue', () => {
     )
   })
 
+  it('rejects impossible month-name calendar dates', () => {
+    expect(
+      normalizeDateCellValue('February 29, 2025 2:30 AM', { timezone: 'America/New_York' })
+    ).toBeNull()
+    expect(
+      normalizeDateCellValue('April 31, 2026 4:04 PM', { timezone: 'America/New_York' })
+    ).toBeNull()
+  })
+
+  it('accepts valid leap-day month-name wall clocks in either date order', () => {
+    expect(
+      normalizeDateCellValue('February 29, 2024 4:04 PM', { timezone: 'America/New_York' })
+    ).toBe('2024-02-29T16:04:00-05:00')
+    expect(normalizeDateCellValue('29 Feb 2024 4:04 PM', { timezone: 'America/New_York' })).toBe(
+      '2024-02-29T16:04:00-05:00'
+    )
+  })
+
   it.each([
-    ['America/New_York', '2026-11-01 01:30:00', '2026-11-01T01:30:00-04:00'],
+    ['America/New_York', '2026-11-01 01:30:00', '2026-11-01T01:30:00-05:00'],
     ['America/New_York', '2026-03-08 02:30:00', '2026-03-08T02:30:00-05:00'],
     ['Asia/Kathmandu', '2026-06-15 09:00:00', '2026-06-15T09:00:00+05:45'],
     ['Australia/Lord_Howe', '2026-06-15 09:00:00', '2026-06-15T09:00:00+10:30'],

apps/sim/lib/table/dates.ts

@@ -38,6 +38,31 @@ const WALL_INSTANT_PATTERN =
 const LOCALIZED_WALL_CLOCK_PATTERN =
   /^(\d{1,2})\/(\d{1,2})\/(\d{4})[ ,]+(\d{1,2}):(\d{2})(?::(\d{2}))?(?:\s*(AM|PM))?$/i
 
+const MONTH_NAME_PATTERN =
+  'Jan(?:uary)?|Feb(?:ruary)?|Mar(?:ch)?|Apr(?:il)?|May|Jun(?:e)?|Jul(?:y)?|Aug(?:ust)?|Sep(?:t(?:ember)?)?|Oct(?:ober)?|Nov(?:ember)?|Dec(?:ember)?'
+const MONTH_FIRST_DATE_PATTERN = new RegExp(
+  `\\b(${MONTH_NAME_PATTERN})\\s+(\\d{1,2})(?:,)?\\s+(\\d{4})\\b`,
+  'i'
+)
+const DAY_FIRST_DATE_PATTERN = new RegExp(
+  `\\b(\\d{1,2})\\s+(${MONTH_NAME_PATTERN})(?:,)?\\s+(\\d{4})\\b`,
+  'i'
+)
+const MONTH_BY_ABBREVIATION: Record<string, number> = {
+  JAN: 1,
+  FEB: 2,
+  MAR: 3,
+  APR: 4,
+  MAY: 5,
+  JUN: 6,
+  JUL: 7,
+  AUG: 8,
+  SEP: 9,
+  OCT: 10,
+  NOV: 11,
+  DEC: 12,
+}
+
 /**
  * Legacy shape: old CSV imports stored date-only columns as UTC-midnight
  * instants. Treated as calendar dates so historical rows render as pure days
@@ -183,11 +208,46 @@ function parseLocalizedWallClock(match: RegExpMatchArray): string | null {
   )
 }
 
+interface CalendarFields {
+  year: number
+  month: number
+  day: number
+}
+
+/** Extracts literal calendar fields from supported month-name date forms. */
+function extractMonthNameCalendar(value: string): CalendarFields | null {
+  const monthFirst = value.match(MONTH_FIRST_DATE_PATTERN)
+  if (monthFirst) {
+    return {
+      year: Number(monthFirst[3]),
+      month: MONTH_BY_ABBREVIATION[monthFirst[1].slice(0, 3).toUpperCase()],
+      day: Number(monthFirst[2]),
+    }
+  }
+  const dayFirst = value.match(DAY_FIRST_DATE_PATTERN)
+  if (!dayFirst) return null
+  return {
+    year: Number(dayFirst[3]),
+    month: MONTH_BY_ABBREVIATION[dayFirst[2].slice(0, 3).toUpperCase()],
+    day: Number(dayFirst[1]),
+  }
+}
+
 /** Recovers broader naive `Date.parse` inputs without consulting the runtime timezone. */
 function parseNaiveWallClockAsUtc(value: string): string | null {
+  const calendar = extractMonthNameCalendar(value)
+  if (calendar && !isValidCalendarDay(calendar.year, calendar.month, calendar.day)) return null
   const ms = Date.parse(`${value} UTC`)
   if (Number.isNaN(ms)) return null
   const parsed = new Date(ms)
+  if (
+    calendar &&
+    (parsed.getUTCFullYear() !== calendar.year ||
+      parsed.getUTCMonth() + 1 !== calendar.month ||
+      parsed.getUTCDate() !== calendar.day)
+  ) {
+    return null
+  }
   return `${toUtcCalendarDate(parsed)}T${pad(parsed.getUTCHours())}:${pad(parsed.getUTCMinutes())}:${pad(parsed.getUTCSeconds())}`
 }