Fake the clock and the network in unit tests

How to do it

Pass the clock and the random source in as arguments, or use fake timers such as vi.useFakeTimers() in Vitest or jest.useFakeTimers() in Jest. In Python, freeze time with a library such as time-machine. Restore the system clock after each test, even when it fails. Seed every random source. Block network calls in unit tests, for example with pytest-socket.

Example

import { vi, test, expect, afterEach } from 'vitest';

afterEach(() => { vi.useRealTimers(); });

test('a trial ends after 14 days', () => {
  vi.useFakeTimers();
  vi.setSystemTime(new Date('2026-03-01T00:00:00Z'));
  const trial = startTrial();
  vi.setSystemTime(new Date('2026-03-15T00:00:01Z'));
  expect(trial.isActive()).toBe(false);
});

# pytest with time-machine
import time_machine

def test_a_trial_ends_after_14_days():
    with time_machine.travel("2026-03-01T00:00:00Z", tick=False) as traveller:
        trial = start_trial()
        traveller.move_to("2026-03-15T00:00:01Z")
        assert not trial.is_active()

How to check

Run the suite in two far-apart time zones, such as TZ=Pacific/Kiritimati and TZ=Pacific/Pago_Pago. Run it with the date moved a year ahead, such as with faketime '+365d'. Then run it with the network off, such as in a container started with --network none. All runs pass.