fix(core): make day conversion deterministic

packages/core/src/__tests__/time.test.ts

@@ -16,6 +16,8 @@
 import { type Timestamp } from '../classes'
 import { getDay, convertToDay } from '..'
 
+const dayTimeZone = 'Europe/London'
+
 const supportedTimezones: string[] = [
   'Europe/Andorra',
   'Asia/Dubai',
@@ -348,18 +350,22 @@ const supportedTimezones: string[] = [
   'Africa/Johannesburg'
 ]
 
+function localeDate (date: Date, timezone: string): string {
+  return date.toLocaleDateString('en-US', { timeZone: timezone })
+}
+
 function testGetDay (date: Date, timezone: string): void {
   const timestamp: Timestamp = getDay(date.getTime())
   const convertedDate: Date = new Date(timestamp)
-  const originalLocaleDate: string = date.toLocaleDateString('en-US', { timeZone: 'Europe/London' })
-  const convertedLocaleDate: string = convertedDate.toLocaleDateString('en-US', { timeZone: timezone })
+  const originalLocaleDate: string = localeDate(date, dayTimeZone)
+  const convertedLocaleDate: string = localeDate(convertedDate, timezone)
   expect(convertedLocaleDate).toEqual(originalLocaleDate)
 }
 
 function testConvertToDay (date: Date, timezone: string): void {
   const convertedDate: Date = convertToDay(date)
-  const originalLocaleDate: string = date.toLocaleDateString('en-US', { timeZone: 'Europe/London' })
-  const convertedLocaleDate: string = convertedDate.toLocaleDateString('en-US', { timeZone: timezone })
+  const originalLocaleDate: string = localeDate(date, dayTimeZone)
+  const convertedLocaleDate: string = localeDate(convertedDate, timezone)
   expect(convertedLocaleDate).toEqual(originalLocaleDate)
 }
 
@@ -389,10 +395,23 @@ describe('time', () => {
     'dates are matched for time [h: %p, m: %p, s: %p, ms: %p]',
     (hours: number, minutes: number, seconds: number, milliSeconds: number) => {
       const date: Date = new Date()
-      const expectedDay: number = date.getDate()
       date.setHours(hours, minutes, seconds, milliSeconds)
       const convertedDate: Date = convertToDay(date)
-      expect(convertedDate.getDate()).toEqual(expectedDay)
+      expect(localeDate(convertedDate, dayTimeZone)).toEqual(localeDate(date, dayTimeZone))
+      expect(convertedDate.getUTCHours()).toEqual(12)
+      expect(convertedDate.getUTCMinutes()).toEqual(0)
+      expect(convertedDate.getUTCSeconds()).toEqual(0)
+      expect(convertedDate.getUTCMilliseconds()).toEqual(0)
     }
   )
+
+  it.each([
+    [new Date('2025-01-25T00:00:00.000Z'), '2025-01-25T12:00:00.000Z'],
+    [new Date('2024-06-12T23:30:00.000Z'), '2024-06-13T12:00:00.000Z']
+  ])('normalizes the London calendar day to noon UTC for %p', (date: Date, expected: string) => {
+    const expectedTimestamp = Date.parse(expected)
+
+    expect(convertToDay(date).getTime()).toEqual(expectedTimestamp)
+    expect(getDay(date.getTime())).toEqual(expectedTimestamp)
+  })
 })

packages/core/src/time.ts

@@ -14,20 +14,48 @@
 //
 import { type Timestamp } from './classes'
 
+const dayTimeZone = 'Europe/London'
+const dayFormatter = new Intl.DateTimeFormat('en-US', {
+  timeZone: dayTimeZone,
+  year: 'numeric',
+  month: 'numeric',
+  day: 'numeric'
+})
+
+function parseCalendarPart (partName: string, partValue: string): number {
+  const value = Number(partValue)
+  if (!Number.isInteger(value)) {
+    throw new Error(`Unable to parse ${dayTimeZone} calendar ${partName}`)
+  }
+  return value
+}
+
 export function getDay (time: Timestamp): Timestamp {
   const date: Date = new Date(time)
   return convertToDay(date).getTime()
 }
 
 export function convertToDay (date: Date): Date {
-  const originalDay: number = date.getDate()
-  const convertedDate: Date = new Date(date)
-  // Set 12 AM UTC time, since it will be the same day in most timezones
-  convertedDate.setUTCHours(12, 0, 0, 0)
-  if (convertedDate.getDate() !== originalDay) {
-    convertedDate.setDate(originalDay)
+  let year: number | undefined
+  let month: number | undefined
+  let day: number | undefined
+
+  for (const part of dayFormatter.formatToParts(date)) {
+    if (part.type === 'year') {
+      year = parseCalendarPart(part.type, part.value)
+    } else if (part.type === 'month') {
+      month = parseCalendarPart(part.type, part.value)
+    } else if (part.type === 'day') {
+      day = parseCalendarPart(part.type, part.value)
+    }
+  }
+
+  if (year === undefined || month === undefined || day === undefined) {
+    throw new Error(`Unable to resolve ${dayTimeZone} calendar day`)
   }
-  return convertedDate
+
+  // Set noon UTC, since it will be the same day in most timezones.
+  return new Date(Date.UTC(year, month - 1, day, 12, 0, 0, 0))
 }
 
 export function getHour (time: Timestamp): Timestamp {