Due-date tests fail at night here and all day in the Tokyo office

Problem Description

test/dueDate.test.js is green in CI and green for most of us most of the time. Two things break it:

• Our nightly 23:45 pipeline run fails an invoice due in thirty minutes is flagged due today with expected 'Upcoming' to equal 'Due today'. The 09:00 run of the same commit is green.
• A colleague who joined the Tokyo office fails an invoice due later today is flagged due today on every run, all day, on a clean checkout.

Nobody else can reproduce the Tokyo failure, and nobody in Tokyo can reproduce the green run. Re-running the nightly job at 23:55 usually fails too; re-running it at 00:30 passes.

src/dueDate.js is behaving as specified: invoices are due on the customer's local calendar day, and product has confirmed that is what they want. The bug report is about the tests, not the rule.

Output Specification

1. Fix test/dueDate.test.js so all three tests pass on any machine at any time of day. A reviewer will run the file with the machine configured for Tokyo, for Los Angeles, and for UTC, and at 23:50 local as well as at midday.
2. Keep all three tests and the behaviour each one describes. Do not modify src/dueDate.js.
3. Write date-handling-notes.md: what varies between a passing run and a failing one, why one test fails only in a narrow window and the other fails constantly for one colleague, and the rule for writing the next test in this file.

Run node --test before you finish; it must pass.

Input Files

Extract the following files before beginning.

=============== FILE: package.json =============== { "name": "billing-web", "version": "4.6.0", "private": true, "scripts": { "test": "node --test" } }

=============== FILE: src/dueDate.js =============== 'use strict';

// Invoices are due on the customer's local calendar day. function isDueToday(dueAtIso, now = new Date()) { const due = new Date(dueAtIso); return ( due.getFullYear() === now.getFullYear() && due.getMonth() === now.getMonth() && due.getDate() === now.getDate() ); }

function dueLabel(dueAtIso, now = new Date()) { if (isDueToday(dueAtIso, now)) { return 'Due today'; } return new Date(dueAtIso) < now ? 'Overdue' : 'Upcoming'; }

module.exports = { isDueToday, dueLabel };

=============== FILE: test/dueDate.test.js =============== 'use strict';

const test = require('node:test'); const assert = require('node:assert/strict'); const { isDueToday, dueLabel } = require('../src/dueDate');

const today = new Date().toISOString().slice(0, 10);

test('an invoice due later today is flagged due today', () => { assert.equal(isDueToday(${today}T23:30:00Z), true); });

test('an invoice due in thirty minutes is flagged due today', () => { const dueAt = new Date(Date.now() + 30 * 60 * 1000).toISOString();

assert.equal(dueLabel(dueAt), 'Due today'); });

test('an invoice from last week is overdue', () => { const dueAt = new Date(Date.now() - 7 * 24 * 60 * 60 * 1000).toISOString();

assert.equal(dueLabel(dueAt), 'Overdue'); });