Dates and times – Test runner

bun:test lets you change what time it is in your tests.

This works with any of the following:

Date.now
new Date()
new Intl.DateTimeFormat().format()

Timers are not impacted yet, but may be in a future release of Bun.

`setSystemTime`

To change the system time, use setSystemTime:

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

beforeAll(() => {
  setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
});

test("it is 2020", () => {
  expect(new Date().getFullYear()).toBe(2020);
});

To support existing tests that use Jest's useFakeTimers and useRealTimers, you can use useFakeTimers and useRealTimers:

test("just like in jest", () => {
  jest.useFakeTimers();
  jest.setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
  expect(new Date().getFullYear()).toBe(2020);
  jest.useRealTimers();
  expect(new Date().getFullYear()).toBeGreaterThan(2020);
});

test("unlike in jest", () => {
  const OriginalDate = Date;
  jest.useFakeTimers();
  if (typeof Bun === "undefined") {
    // In Jest, the Date constructor changes
    // That can cause all sorts of bugs because suddenly Date !== Date before the test.
    expect(Date).not.toBe(OriginalDate);
    expect(Date.now).not.toBe(OriginalDate.now);
  } else {
    // In bun:test, Date constructor does not change when you useFakeTimers
    expect(Date).toBe(OriginalDate);
    expect(Date.now).toBe(OriginalDate.now);
  }
});

Timers — Note that we have not implemented builtin support for mocking timers yet, but this is on the roadmap.

Reset the system time

To reset the system time, pass no arguments to setSystemTime:

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

test("it was 2020, for a moment.", () => {
  // Set it to something!
  setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
  expect(new Date().getFullYear()).toBe(2020);

  // reset it!
  setSystemTime();

  expect(new Date().getFullYear()).toBeGreaterThan(2020);
});

Set the time zone

To change the time zone, either pass the $TZ environment variable to bun test.

TZ=America/Los_Angeles bun test

Or set process.env.TZ at runtime:

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

test("Welcome to California!", () => {
  process.env.TZ = "America/Los_Angeles";
  expect(new Date().getTimezoneOffset()).toBe(420);
  expect(new Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(
    "America/Los_Angeles",
  );
});

test("Welcome to New York!", () => {
  // Unlike in Jest, you can set the timezone multiple times at runtime and it will work.
  process.env.TZ = "America/New_York";
  expect(new Date().getTimezoneOffset()).toBe(240);
  expect(new Intl.DateTimeFormat().resolvedOptions().timeZone).toBe(
    "America/New_York",
  );
});