Fix a bug when a year is a 2-digit number in toDateFromClockTime

Author: fabon-f

src/datetime/_createDateFromClockTime.ts

@@ -0,0 +1,30 @@
+import type { GenericDateConstructor } from "../types.js";
+
+export function createDateFromClockTime<DateType extends Date>(
+	DateConstructor: GenericDateConstructor<DateType>,
+	year: number,
+	month: number,
+	day = 1,
+	hour = 0,
+	minute = 0,
+	second = 0,
+	millisecond = 0,
+): DateType {
+	if (year >= 0 && year <= 99) {
+		// assumption: there is no time zone transition in ancient times
+		// additionally (just in case) 2024-01-05T00:00:00 should exist in all time zones
+		const date = new DateConstructor(2024, 0, 5);
+		date.setFullYear(year, month - 1, day);
+		date.setHours(hour, minute, second, millisecond);
+		return date;
+	}
+	return new DateConstructor(
+		year,
+		month - 1,
+		day,
+		hour,
+		minute,
+		second,
+		millisecond,
+	);
+}

src/datetime/toDateFromClockTime.test.ts

@@ -141,3 +141,15 @@ test("date constructor type", () => {
 		UTCDate,
 	);
 });
+
+test("2-digit year", () => {
+	expect(
+		toDateFromClockTime(Temporal.PlainDateTime.from("0050-01-01")),
+	).toEqual(new UTCDate("0050-01-01T00:00:00Z"));
+	expect(toDateFromClockTime(Temporal.PlainDate.from("0050-01-01"))).toEqual(
+		new UTCDate("0050-01-01T00:00:00Z"),
+	);
+	expect(
+		toDateFromClockTime(Temporal.PlainYearMonth.from("0050-01-01")),
+	).toEqual(new UTCDate("0050-01-01T00:00:00Z"));
+});

src/datetime/toDateFromClockTime.ts

@@ -8,6 +8,7 @@ import {
 	isZonedDateTime,
 } from "../type-utils.js";
 import type { GenericDateConstructor, Temporal } from "../types.js";
+import { createDateFromClockTime } from "./_createDateFromClockTime.js";
 
 function parseIsoString(date: string) {
 	const res = /^(\d{4,}|[+-]\d{6})-(\d{2})-(\d{2})/.exec(date);
@@ -25,6 +26,40 @@ function parseIsoString(date: string) {
 	};
 }
 
+// function to bypass an enigmatic TypeScript error "could be instantiated with a different subtype of constraint"
+function createDate<DateType extends Date>(
+	DateConstructor: GenericDateConstructor<DateType> | undefined,
+	year: number,
+	month: number,
+	day = 1,
+	hour = 0,
+	minute = 0,
+	second = 0,
+	millisecond = 0,
+) {
+	return DateConstructor ?
+			createDateFromClockTime(
+				DateConstructor,
+				year,
+				month,
+				day,
+				hour,
+				minute,
+				second,
+				millisecond,
+			)
+		:	createDateFromClockTime(
+				UTCDate,
+				year,
+				month,
+				day,
+				hour,
+				minute,
+				second,
+				millisecond,
+			);
+}
+
 /**
  * Returns `Date` which represents clock (local) time of given temporal object,
  * dropping timezone and calendar information.
@@ -79,22 +114,22 @@ export function toDateFromClockTime<DateType extends Date>(
 		| Temporal.PlainMonthDay,
 	DateConstructor?: GenericDateConstructor<DateType>,
 ) {
-	const DateConstructorFunction = DateConstructor ?? UTCDate;
 	if (isPlainYearMonth(dateTime)) {
 		const pd = dateTime.toPlainDate({ day: 1 }).withCalendar("iso8601");
-		return new DateConstructorFunction(pd.year, pd.month - 1, pd.day);
+		return createDate(DateConstructor, pd.year, pd.month, pd.day);
 	}
 	if (isPlainMonthDay(dateTime)) {
 		if (dateTime.calendarId === "iso8601") {
 			const pd = dateTime.toPlainDate({ year: 1972 });
-			return new DateConstructorFunction(pd.year, pd.month - 1, pd.day);
+			return createDate(DateConstructor, pd.year, pd.month, pd.day);
 		}
 		const { year, month, day } = parseIsoString(dateTime.toString());
-		return new DateConstructorFunction(year, month - 1, day);
+		return createDate(DateConstructor, year, month, day);
 	}
 	if (isPlainTime(dateTime)) {
 		// Set default date to 2000-01-01
-		return new DateConstructorFunction(
+		return createDate(
+			DateConstructor,
 			2000,
 			0,
 			1,
@@ -109,9 +144,10 @@ export function toDateFromClockTime<DateType extends Date>(
 			dateTime.toPlainDateTime().withCalendar("iso8601")
 		: isPlainDate(dateTime) ? dateTime.toPlainDateTime().withCalendar("iso8601")
 		: dateTime.withCalendar("iso8601");
-	return new DateConstructorFunction(
+	return createDate(
+		DateConstructor,
 		plainDateTime.year,
-		plainDateTime.month - 1,
+		plainDateTime.month,
 		plainDateTime.day,
 		plainDateTime.hour,
 		plainDateTime.minute,