Bun Set the system time in Bun's 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.