How do I test code that depends on the current date?

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How do I test code that depends on the current date?

asked 1 month ago

396

I have a function returning "3 days ago" style strings. The tests pass today and fail next month, which makes CI unreliable.

1export function relativeTime(date: Date) {
2  const diff = Date.now() - date.getTime()
3  // ...
4}

1export function relativeTime(date: Date) {
2  const diff = Date.now() - date.getTime()
3  // ...
4}

testing

javascript

typescript

2 Answers

Kenji Tanaka

• answered 1 month ago

Freeze the clock. Every major test runner has fake timers now:

1beforeEach(() => {
2  vi.useFakeTimers()
3  vi.setSystemTime(new Date("2026-01-15T12:00:00Z"))
4})
5
6afterEach(() => vi.useRealTimers())

1beforeEach(() => {
2  vi.useFakeTimers()
3  vi.setSystemTime(new Date("2026-01-15T12:00:00Z"))
4})
5
6afterEach(() => vi.useRealTimers())

The alternative, which is better if you can afford the refactor, is to stop reaching for the global clock at all:

1export function relativeTime(date: Date, now: Date = new Date()) { ... }

1export function relativeTime(date: Date, now: Date = new Date()) { ... }

Now the test passes an explicit now and needs no framework magic. Injected time also makes the function easier to reason about — it becomes pure.

Oliver Bennett

• answered 1 month ago

One gotcha with fake timers: if any code under test awaits a real promise chain, remember to advance the timers and flush microtasks, otherwise you get mysterious hangs.

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