Mocking current time for Date

mockDate.js

describe('...', () => {
  const RealDate = Date;

  function mockDate(isoDate) {
    global.Date = class extends RealDate {
      constructor(...args) {
        if (args.length) return new RealDate(...args);
        return new RealDate(isoDate);
      }
    };
  }

  afterEach(() => {
    global.Date = RealDate;
  });

  it('...', () => {
    mockDate('2018-01-01');
    // rest of the code
  });
})