Guides Test Runner

Set the system time in Bun's test runner

Bun's test runner supports setting the system time programmatically with the setSystemTime function.

import { test, expect, setSystemTime } from "bun:test";

test("party like it's 1999", () => {
  const date = new Date("1999-01-01T00:00:00.000Z");
  setSystemTime(date); // it's now January 1, 1999

  const now = new Date();
  expect(now.getFullYear()).toBe(1999);
  expect(now.getMonth()).toBe(0);
  expect(now.getDate()).toBe(1);
});

The setSystemTime function is commonly used on conjunction with Lifecycle Hooks to configure a testing environment with a deterministic "fake clock".

import { test, expect, beforeAll, setSystemTime } from "bun:test";

beforeAll(() => {
  const date = new Date("1999-01-01T00:00:00.000Z");
  setSystemTime(date); // it's now January 1, 1999
});

// tests...

To reset the system clock to the actual time, call setSystemTime with no arguments.

import { test, expect, beforeAll, setSystemTime } from "bun:test";

setSystemTime(); // reset to actual time

See Docs > Test Runner > Date and time for complete documentation on mocking with the Bun test runner.

Migrate from Jest

Mock functions

Set the system time in Bun's test runner

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