"TemporalDeterminismAuditor: When the Wall Clock Is a Bug"

Wall-clock nondeterminism: simulated data stamped with .now, and tests that assert on measured elapsed time.

---

The wall clock is the other great enemy of a reproducible test suite. Two shapes cause most of the trouble, and this checker catches both — using the AST to tell them apart from legitimate uses.

What it catches

Simulated sources stamping .now. A data generator, fixture, or mock that stamps records with Date.now produces different output every run. That’s fine for real telemetry; it’s a bug in a simulated source that’s supposed to be reproducible.

// ❌ a synthetic fixture that isn't reproducible
func makeSample() -> Sample { Sample(timestamp: .now, value: 42) }

// ✅ inject the clock so the fixture is deterministic
func makeSample(at time: Date) -> Sample { Sample(timestamp: time, value: 42) }

Tests asserting on measured elapsed time. A test that does let t = Date(); …; XCTAssert(Date().timeIntervalSince(t) < 0.1) is asserting on how fast the test machine is. It passes on your laptop and fails in CI under load. The rule flags tests that assert on wall-clock elapsed time and points you at logical clocks or injected time instead.

Precise about which rule runs where

The checker is context-aware: a path containing /Tests/ runs the elapsed-time-assertion rule; production paths run the simulated-source rule. And it doesn’t flag its own timing — the auditor measures its runtime with ContinuousClock, which is exactly the right, non-flagged way to time an operation. It flags the patterns that make behavior non-reproducible, not every mention of time.

Together with StochasticDeterminismAuditor, it enforces the rule that makes the flip-detector and stress-mode of the test runner meaningful: a suite that’s deterministic to begin with, so a flip is a race, not the clock.

Try it

quality-gate --check temporal-determinism

---

Source: github.com/jpurnell/quality-gate-swift