sendou.ink/app/utils/dates.test.ts

import { afterAll, beforeAll, describe, expect, test } from "vitest";
import { weekNumberToDate, weekNumberToDateRange } from "./dates";

describe("weekNumberToDate", () => {
	// the bug only manifests west of UTC
	const originalTimezone = process.env.TZ;
	beforeAll(() => {
		process.env.TZ = "America/Los_Angeles";
	});
	afterAll(() => {
		process.env.TZ = originalTimezone;
	});

	test("returns the Monday of the ISO week regardless of server timezone", () => {
		// ISO week 1 of 2024 starts on Monday 2024-01-01
		const start = weekNumberToDate({ week: 1, year: 2024 });

		expect(start.toISOString().slice(0, 10)).toBe("2024-01-01");
	});

	test("returns the Sunday of the ISO week regardless of server timezone", () => {
		// ISO week 1 of 2024 ends on Sunday 2024-01-07
		const end = weekNumberToDate({ week: 1, year: 2024, position: "end" });

		expect(end.toISOString().slice(0, 10)).toBe("2024-01-07");
	});
});

describe("weekNumberToDateRange", () => {
	// a timezone west of UTC observing DST
	const originalTimezone = process.env.TZ;
	beforeAll(() => {
		process.env.TZ = "America/New_York";
	});
	afterAll(() => {
		process.env.TZ = originalTimezone;
	});

	const SEVEN_DAYS_MS = 7 * 24 * 60 * 60 * 1000;

	test("spans exactly seven days even when the week contains a DST transition", () => {
		// US spring-forward Sunday 2025-03-09 falls inside ISO week 10 (Mon 03-03 .. Mon 03-10)
		const { startTime, endTime } = weekNumberToDateRange({
			week: 10,
			year: 2025,
		});

		expect(endTime.getTime() - startTime.getTime()).toBe(SEVEN_DAYS_MS);
	});

	test("spans exactly seven days for an ordinary week", () => {
		const { startTime, endTime } = weekNumberToDateRange({
			week: 20,
			year: 2025,
		});

		expect(endTime.getTime() - startTime.getTime()).toBe(SEVEN_DAYS_MS);
	});
});