## TL;DR

Tests that read the real clock break at boundaries. Inject the clock (fake timers, time parameters) so tests control time, and never assert on absolute "now" in test logic.

## Error

```text
(Not an error; a flake pattern. Symptom: failures cluster around midnight, DST changes, or month ends.)
```

## Steps

1. Find clock reads: `Date.now()`, `new Date()`, `time.Now()` in test and app code paths. Expected: the time dependencies listed.
2. Replace with injected time: fake timers in tests, a clock parameter in app code. Expected: tests set the time explicitly.
3. Test the boundaries deliberately: midnight, DST transition, leap day, month end. Expected: boundary cases covered by explicit tests.
4. Pin the timezone in CI (`TZ=UTC`) and in test setup. Expected: no environment-dependent time.
5. Re-run across a DST boundary date. Expected: green.

## When to use

- Failures correlate with calendar boundaries.
- Tests assert on "today", "now", or durations.

## When not to use

- Race conditions (timing, not calendar).
- Performance timing assertions.

## Tool compatibility

- Fake timers per framework; `TZ` env var.

## Variant phrasings

### Flaky test fails at midnight

The classic symptom; clock injection is the fix.

### DST test failures

The seasonal variant; test the transition explicitly.

## Why it happens

Real time is an uncontrolled input. Boundaries change date math, and tests that assume "today" break when today changes mid-run.

## Edge cases

- Fake timers break animation frames and some libraries; scope them narrowly.
- Database `now()` calls need DB-level time control or tolerance windows.
- Never assert exact timestamps from the real clock; assert relative ordering.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_dDbvRn0E7WptFF3uKWgEYQ