name	effect-clock-patterns
description	Effect Clock for injectable, testable time. Never use new Date() directly.
allowed-tools	Read, Write, Edit
token-budget	800
version	1.0.0

Effect Clock Patterns

Why Not new Date()?

// UNTESTABLE
const createdAt = new Date();

Clock Service

import { Clock, Effect } from "effect";

// Helper (put in lib/clock.ts)
export const currentTimestamp = Effect.map(
  Clock.currentTimeMillis,
  (ms) => new Date(ms)
);

// Usage
const program = Effect.gen(function* () {
  const now = yield* currentTimestamp;
  yield* db.update(records).set({ updatedAt: now });
});

Testing

import { TestClock, Effect } from "effect";

const test = Effect.gen(function* () {
  yield* TestClock.setTime(new Date("2024-06-15").getTime());
  const result = yield* myTimeDependentCode;
  yield* TestClock.adjust("1 hour");
});

Effect.runPromise(test.pipe(Effect.provide(TestClock.layer)));

Migration

Before	After
`new Date()`	`yield* currentTimestamp`
`Date.now()`	`yield* Clock.currentTimeMillis`

effect-clock-patterns

Install Skill

SKILL.md

Effect Clock Patterns

Why Not new Date()?

Clock Service

Testing

Migration