fix(timezone): consolidate table wall-clock conversion

Author: j15z

apps/sim/app/workspace/[workspaceId]/tables/[tableId]/utils.ts

@@ -1,7 +1,8 @@
+import { getWallClockParts } from '@/lib/core/utils/timezone'
 import type { ColumnDefinition, JsonValue } from '@/lib/table'
 import type { ColumnType } from '@/lib/table/column-types'
 import { columnTypeById, columnTypeOf } from '@/lib/table/column-types'
-import { formatDateCellDisplay, getWallClockParts, normalizeDateCellValue } from '@/lib/table/dates'
+import { formatDateCellDisplay, normalizeDateCellValue } from '@/lib/table/dates'
 
 /**
  * Pick a fresh "untitled[_N]" name not already taken by `columns`. Used by

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

@@ -1,12 +1,63 @@
 import { describe, expect, it } from 'vitest'
 import {
+  formatInstantInTimeZone,
   getSupportedTimezones,
   getTimezoneOptions,
+  getWallClockParts,
   wallClockNow,
   zonedClockDate,
   zonedWallClockToUtc,
+  zonedWallClockWithOffset,
 } from './timezone'
 
+describe('formatInstantInTimeZone', () => {
+  it.each([
+    ['UTC', '2026-06-15T00:15:30Z', '2026-06-15T00:15:30Z'],
+    ['America/Los_Angeles', '2026-06-15T00:15:30Z', '2026-06-14T17:15:30-07:00'],
+    ['Asia/Tokyo', '2026-06-15T00:15:30Z', '2026-06-15T09:15:30+09:00'],
+    ['Asia/Kathmandu', '2026-06-15T00:15:30Z', '2026-06-15T06:00:30+05:45'],
+    ['Australia/Lord_Howe', '2026-06-15T00:15:30Z', '2026-06-15T10:45:30+10:30'],
+  ])('formats an instant in %s with its exact offset', (timeZone, iso, expected) => {
+    expect(formatInstantInTimeZone(new Date(iso), timeZone)).toBe(expected)
+  })
+
+  it('distinguishes both copies of an autumn daylight-saving hour', () => {
+    expect(formatInstantInTimeZone(new Date('2026-11-01T05:30:00Z'), 'America/New_York')).toBe(
+      '2026-11-01T01:30:00-04:00'
+    )
+    expect(formatInstantInTimeZone(new Date('2026-11-01T06:30:00Z'), 'America/New_York')).toBe(
+      '2026-11-01T01:30:00-05:00'
+    )
+  })
+
+  it('round-trips the same instant after changing display timezones', () => {
+    const instant = new Date('2026-11-01T06:30:00Z')
+    for (const timeZone of [
+      'UTC',
+      'America/Los_Angeles',
+      'America/New_York',
+      'Asia/Kathmandu',
+      'Australia/Lord_Howe',
+    ]) {
+      const editable = formatInstantInTimeZone(instant, timeZone)
+      expect(new Date(editable).getTime()).toBe(instant.getTime())
+    }
+  })
+})
+
+describe('getWallClockParts', () => {
+  it('returns the calendar fields of an instant in the requested timezone', () => {
+    expect(getWallClockParts(new Date('2026-06-15T00:15:30Z'), 'America/Los_Angeles')).toEqual({
+      year: 2026,
+      month: 6,
+      day: 14,
+      hour: 17,
+      minute: 15,
+      second: 30,
+    })
+  })
+})
+
 describe('zonedWallClockToUtc', () => {
   it('treats a UTC wall-clock as the same instant', () => {
     expect(zonedWallClockToUtc('2026-06-15T09:00', 'UTC').toISOString()).toBe(
@@ -48,11 +99,57 @@ describe('zonedWallClockToUtc', () => {
   })
 
   it('resolves a spring-forward gap wall-clock forward by the DST shift', () => {
-    // 2026-03-08 02:00–02:59 does not exist in America/New_York (EST→EDT).
-    expect(zonedWallClockToUtc('2026-03-08T02:30', 'America/New_York').toISOString()).toBe(
-      '2026-03-08T07:30:00.000Z'
-    )
+    const instant = zonedWallClockToUtc('2026-03-08T02:30', 'America/New_York')
+    const stampedWallClock = zonedWallClockWithOffset('2026-03-08T02:30', 'America/New_York')
+
+    expect(instant.toISOString()).toBe('2026-03-08T07:30:00.000Z')
+    expect(stampedWallClock).toBe('2026-03-08T02:30-05:00')
+    expect(new Date(stampedWallClock).toISOString()).toBe(instant.toISOString())
   })
+
+  it.each([
+    [
+      'Europe/Berlin',
+      '2026-03-29T02:30',
+      '2026-03-29T01:30:00.000Z',
+      '2026-03-29T03:30:00+02:00',
+      '2026-03-29T02:30+01:00',
+    ],
+    [
+      'Australia/Lord_Howe',
+      '2026-10-04T02:15',
+      '2026-10-03T15:45:00.000Z',
+      '2026-10-04T02:45:00+11:00',
+      '2026-10-04T02:15+10:30',
+    ],
+  ])(
+    'resolves an east-of-UTC spring-forward gap in %s to the first compatible wall-clock',
+    (timeZone, wallClock, expectedInstant, expectedRenderedWallClock, expectedStampedWallClock) => {
+      const instant = zonedWallClockToUtc(wallClock, timeZone)
+      const stampedWallClock = zonedWallClockWithOffset(wallClock, timeZone)
+
+      expect(instant.toISOString()).toBe(expectedInstant)
+      expect(formatInstantInTimeZone(instant, timeZone)).toBe(expectedRenderedWallClock)
+      expect(stampedWallClock).toBe(expectedStampedWallClock)
+      expect(new Date(stampedWallClock).toISOString()).toBe(expectedInstant)
+    }
+  )
+
+  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'],
+  ])(
+    'keeps the earlier instant for an ambiguous fall-back wall-clock in %s',
+    (timeZone, wallClock, expectedInstant, expectedOffset) => {
+      const instant = zonedWallClockToUtc(wallClock, timeZone)
+      const stampedWallClock = zonedWallClockWithOffset(wallClock, timeZone)
+
+      expect(instant.toISOString()).toBe(expectedInstant)
+      expect(stampedWallClock).toBe(`${wallClock}${expectedOffset}`)
+      expect(new Date(stampedWallClock).toISOString()).toBe(expectedInstant)
+    }
+  )
 })
 
 describe('wallClockNow', () => {

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

@@ -23,6 +23,41 @@ const COMMON_TIMEZONES = [
   'Australia/Sydney',
 ]
 
+/** A wall-clock reading of an instant in some timezone. */
+export interface WallClockParts {
+  year: number
+  /** 1-based month. */
+  month: number
+  day: number
+  hour: number
+  minute: number
+  second: number
+}
+
+function pad(value: number): string {
+  return String(value).padStart(2, '0')
+}
+
+/** RFC 3339 offset suffix: `Z` for zero, else `±HH:MM`. */
+export function formatUtcOffsetSuffix(offsetMinutes: number): string {
+  if (offsetMinutes === 0) return 'Z'
+  const sign = offsetMinutes > 0 ? '+' : '-'
+  const absoluteMinutes = Math.abs(offsetMinutes)
+  return `${sign}${pad(Math.floor(absoluteMinutes / 60))}:${pad(absoluteMinutes % 60)}`
+}
+
+function offsetMsFromWallClock(instant: Date, wall: WallClockParts): number {
+  const wallAsUtc = Date.UTC(
+    wall.year,
+    wall.month - 1,
+    wall.day,
+    wall.hour,
+    wall.minute,
+    wall.second
+  )
+  return wallAsUtc - instant.getTime()
+}
+
 /** The IANA timezone the current runtime resolves to (e.g. `America/New_York`). */
 export function getBrowserTimezone(): string {
   return Intl.DateTimeFormat().resolvedOptions().timeZone
@@ -116,22 +151,57 @@ export function getTimezoneOptions(): TimezoneOption[] {
 }
 
 /**
- * An instant's wall-clock time in `timeZone` as a naive `yyyy-MM-ddTHH:mm`
- * string. Lets callers reason about a user's local date/time without UTC — e.g.
- * to recover the local date/time a stored task instant represents in its zone.
+ * The wall-clock fields of `instant` in `timeZone`, or in the runtime's local
+ * timezone when omitted.
  */
-export function zonedWallClock(instant: Date, timeZone: string): string {
-  const parts = new Intl.DateTimeFormat('en-CA', {
+export function getWallClockParts(instant: Date, timeZone?: string): WallClockParts {
+  if (!timeZone) {
+    return {
+      year: instant.getFullYear(),
+      month: instant.getMonth() + 1,
+      day: instant.getDate(),
+      hour: instant.getHours(),
+      minute: instant.getMinutes(),
+      second: instant.getSeconds(),
+    }
+  }
+
+  const parts = new Intl.DateTimeFormat('en-US', {
     timeZone,
+    hourCycle: 'h23',
     year: 'numeric',
     month: '2-digit',
     day: '2-digit',
     hour: '2-digit',
     minute: '2-digit',
-    hourCycle: 'h23',
+    second: '2-digit',
   }).formatToParts(instant)
-  const get = (type: string) => parts.find((p) => p.type === type)?.value ?? '00'
-  return `${get('year')}-${get('month')}-${get('day')}T${get('hour')}:${get('minute')}`
+  const get = (type: string) => Number(parts.find((part) => part.type === type)?.value)
+  return {
+    year: get('year'),
+    month: get('month'),
+    day: get('day'),
+    hour: get('hour'),
+    minute: get('minute'),
+    second: get('second'),
+  }
+}
+
+/** Formats an instant as an RFC 3339 wall time in an IANA timezone. */
+export function formatInstantInTimeZone(instant: Date, timeZone: string): string {
+  const wall = getWallClockParts(instant, timeZone)
+  const offsetMinutes = Math.round(offsetMsFromWallClock(instant, wall) / 60_000)
+  return `${wall.year}-${pad(wall.month)}-${pad(wall.day)}T${pad(wall.hour)}:${pad(wall.minute)}:${pad(wall.second)}${formatUtcOffsetSuffix(offsetMinutes)}`
+}
+
+/**
+ * An instant's wall-clock time in `timeZone` as a naive `yyyy-MM-ddTHH:mm`
+ * string. Lets callers reason about a user's local date/time without UTC — e.g.
+ * to recover the local date/time a stored task instant represents in its zone.
+ */
+export function zonedWallClock(instant: Date, timeZone: string): string {
+  const wall = getWallClockParts(instant, timeZone)
+  return `${wall.year}-${pad(wall.month)}-${pad(wall.day)}T${pad(wall.hour)}:${pad(wall.minute)}`
 }
 
 /** The current wall-clock time in `timeZone` as a naive `yyyy-MM-ddTHH:mm` string. */
@@ -156,26 +226,40 @@ export function zonedClockDate(instant: Date, timeZone: string): Date {
 
 /** The UTC offset (ms, east-positive) of `timeZone` at a given instant. */
 function timezoneOffsetMs(instant: Date, timeZone: string): number {
-  const parts = new Intl.DateTimeFormat('en-US', {
-    timeZone,
-    hourCycle: 'h23',
-    year: 'numeric',
-    month: '2-digit',
-    day: '2-digit',
-    hour: '2-digit',
-    minute: '2-digit',
-    second: '2-digit',
-  }).formatToParts(instant)
-  const get = (type: string) => Number(parts.find((p) => p.type === type)?.value)
-  const asUtc = Date.UTC(
-    get('year'),
-    get('month') - 1,
-    get('day'),
-    get('hour'),
-    get('minute'),
-    get('second')
+  return offsetMsFromWallClock(instant, getWallClockParts(instant, timeZone))
+}
+
+interface ZonedWallClockResolution {
+  instant: Date
+  offsetMinutes: number
+}
+
+function resolveZonedWallClock(wallClock: string, timeZone: string): ZonedWallClockResolution {
+  const [datePart, timePart] = wallClock.split('T')
+  const [year, month, day] = datePart.split('-').map(Number)
+  const [hour, minute, second = 0] = timePart.split(':').map(Number)
+  const utcGuess = Date.UTC(year, month - 1, day, hour, minute, second)
+  const dayMs = 24 * 60 * 60 * 1000
+  const offsets = new Set(
+    [-dayMs, 0, dayMs].map((distance) => timezoneOffsetMs(new Date(utcGuess + distance), timeZone))
   )
-  return asUtc - instant.getTime()
+  const candidates = [...offsets].map((offset) => {
+    const instantMs = utcGuess - offset
+    const actualOffset = timezoneOffsetMs(new Date(instantMs), timeZone)
+    return { instantMs, wallClockMs: instantMs + actualOffset }
+  })
+  const exactCandidate = candidates
+    .filter(({ wallClockMs }) => wallClockMs === utcGuess)
+    .sort((a, b) => a.instantMs - b.instantMs)[0]
+  const compatibleCandidate = candidates
+    .filter(({ wallClockMs }) => wallClockMs > utcGuess)
+    .sort((a, b) => a.wallClockMs - b.wallClockMs || a.instantMs - b.instantMs)[0]
+  const chosenCandidate = exactCandidate ?? compatibleCandidate ?? candidates[0]
+  const instantMs = chosenCandidate.instantMs
+  return {
+    instant: new Date(instantMs),
+    offsetMinutes: Math.round((utcGuess - instantMs) / 60_000),
+  }
 }
 
 /**
@@ -185,22 +269,17 @@ function timezoneOffsetMs(instant: Date, timeZone: string): number {
  * 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 later (post-transition) instant is chosen; a
+ * fall-back wall-clock the earlier 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.
  */
 export function zonedWallClockToUtc(wallClock: string, timeZone: string): Date {
-  const [datePart, timePart] = wallClock.split('T')
-  const [year, month, day] = datePart.split('-').map(Number)
-  const [hour, minute, second = 0] = timePart.split(':').map(Number)
-  const utcGuess = Date.UTC(year, month - 1, day, hour, minute, second)
-  const guessOffset = timezoneOffsetMs(new Date(utcGuess), timeZone)
-  const candidate = utcGuess - guessOffset
-  const candidateOffset = timezoneOffsetMs(new Date(candidate), timeZone)
-  if (candidateOffset === guessOffset) return new Date(candidate)
-  const adjusted = utcGuess - candidateOffset
-  return timezoneOffsetMs(new Date(adjusted), timeZone) === candidateOffset
-    ? new Date(adjusted)
-    : new Date(candidate)
+  return resolveZonedWallClock(wallClock, timeZone).instant
+}
+
+/** Stamps a naive wall-clock with the offset selected by the shared timezone resolver. */
+export function zonedWallClockWithOffset(wallClock: string, timeZone: string): string {
+  const { offsetMinutes } = resolveZonedWallClock(wallClock, timeZone)
+  return `${wallClock}${formatUtcOffsetSuffix(offsetMinutes)}`
 }