Set the system time in Bun's test runner

Set the system time in tests with the setSystemTime function from bun:test.

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);
});

Call setSystemTime in a lifecycle hook like beforeAll to give your tests 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 Dates and times.