How to mock Date in Jest tests

If you want to test code against the current date, you're likely to have to mock the JavaScript Date object.

There are two ways to mock the Date object constructor:

• using setSystemTime (starting with Jest 26 and above)
• using spyOn (before Jest 26)

Method 1: setSystemTime

Since Jest 26 you can use Jest setSystemTime to simulate a user changing the system clock while your program is running.

Note that this method works with the new modern fake timer, hence we make use of it, before setting the mock.

To clear the mock, you can call useRealTimers. This function will restore the original implementation of the global Date object.

describe("after jest 26", () => {
  beforeEach(() => {
    const mockedDate = new Date(1999, 10, 1);

    jest.useFakeTimers("modern");
    jest.setSystemTime(mockedDate);
  });

  afterEach(() => {
    jest.useRealTimers();
  });

  test("today is November 1st 1999", () => {
    const today = new Date();
    expect(today.toLocaleDateString("en-US")).toBe("11/1/1999");
  });
});

Method 2: spyOn

If your Jest version is older than Jest 26, you can still mock the Date object using Jest spyOn.

describe("before jest 26", () => {
  let spy;

  beforeEach(() => {
    const mockedDate = new Date(1996, 6, 19);

    spy = jest.spyOn(global, "Date").mockImplementation(() => mockedDate);
  });

  afterEach(() => {
    spy.mockRestore();
  });

  test("today is July 19th 1996", () => {
    const today = new Date();
    expect(today.toLocaleDateString("en-US")).toBe("7/19/1996");
  });
});

Complete code is available here: https://github.com/benoitpaul/js-labs/tree/main/jest-mock-date