effect-clock-patterns

Time-dependent code becomes testable by injecting a Clock service instead of calling new Date or Date.now directly. Effect's Clock.currentTimeMillis provides deterministic timestamps that you control in tests, replacing hardcoded time dependencies with injectable, mockable time sources.

Replace Date.now and new Date calls with yield* currentTimestamp or Clock.currentTimeMillis within Effect code. This yields control of time to the Effect runtime, enabling test clocks to intercept and manipulate timestamps for deterministic test scenarios.

Yes. TestClock provides isolated time manipulation scoped to individual tests without global mocking. You control timestamp advancement, enabling replayable CI scenarios and reliable time-based migrations without affecting other code or tests running concurrently.

Injectable clocks decouple time from implementation details, making migrations safer and tests more composable. TestClock enables deterministic time control per test without global state, preventing test pollution and supporting complex scenarios like time-based migrations with full auditability.

This Skill applies specifically to TypeScript projects using the Effect library. Effect's Clock service and TestClock utilities integrate with Effect's type system and runtime to manage time as a dependency, not as a general-purpose Date replacement.