How do I set a mock date in Jest?

I'm using moment.js to do most of my date logic in a helper file for my React components but I haven't been able to figure out how to mock a date in Jest a la sinon.useFakeTimers().

The Jest docs only speak about timer functions like setTimeout, setInterval etc but don't help with setting a date and then checking that my date functions do what they're meant to do.

Here is some of my JS file:

var moment = require('moment');

var DateHelper = {
  
  DATE_FORMAT: 'MMMM D',
  API_DATE_FORMAT: 'YYYY-MM-DD',
  
  formatDate: function(date) {
    return date.format(this.DATE_FORMAT);
  },

  isDateToday: function(date) {
    return this.formatDate(date) === this.formatDate(moment());
  }
};


module.exports = DateHelper;

and here is what I've set up using Jest:

jest.dontMock('../../../dashboard/calendar/date-helper')
    .dontMock('moment');

describe('DateHelper', function() {
  var DateHelper = require('../../../dashboard/calendar/date-helper'),
      moment = require('moment'),
      DATE_FORMAT = 'MMMM D';

  describe('formatDate', function() {

    it('should return the date formatted as DATE_FORMAT', function() {
      var unformattedDate = moment('2014-05-12T00:00:00.000Z'),
          formattedDate = DateHelper.formatDate(unformattedDate);

      expect(formattedDate).toEqual('May 12');
    });

  });

  describe('isDateToday', function() {

    it('should return true if the passed in date is today', function() {
      var today = moment();

      expect(DateHelper.isDateToday(today)).toEqual(true);
    });
    
  });

});

Now these tests pass because I'm using moment and my functions use moment but it seems a bit unstable and I would like to set the date to a fixed time for the tests.

Any idea on how that could be accomplished?

Since momentjs uses Date internally, you can just overwrite the Date.now function to always return the same moment.

Date.now = jest.fn(() => 1487076708000) //14.02.2017

or

Date.now = jest.fn(() => new Date(Date.UTC(2017, 1, 14)).valueOf())

As of Jest 26 this can be achieved using "modern" fake timers without needing to install any 3rd party modules: https://jestjs.io/blog/2020/05/05/jest-26#new-fake-timers

jest
  .useFakeTimers()
  .setSystemTime(new Date('2020-01-01').getTime());

If you want the fake timers to be active for all tests, you can set timers: 'modern' in your configuration: https://jestjs.io/docs/configuration#timers-string

EDIT: As of Jest 27 modern fake timers is the default, so you can drop the argument to useFakeTimers.

For quick and dirty solution use jest.spyOn for locking time:

let dateNowSpy;

beforeAll(() => {
    // Lock Time
    dateNowSpy = jest.spyOn(Date, 'now').mockImplementation(() => 1487076708000);
});

afterAll(() => {
    // Unlock Time
    dateNowSpy.mockRestore();
});

UPDATE:

For a more robust solution look at timekeeper:

import timekeeper from 'timekeeper';

beforeAll(() => {
    // Lock Time
    timekeeper.freeze(new Date('2014-01-01'));
});

afterAll(() => {
    // Unlock Time
    timekeeper.reset();
});