Set the Date object to assume the current date to be a fixed date. Useful for testing.

fix-current-date.js

function fixCurrentDate(dateString = 'Tue Jan 01 2019 00:00:00 GMT+0800') {
  let RealDate;
  // Make sure we keep a reference to the orginal `Date`
  if (window.Date.isFake && window.Date.RealDate) {
    RealDate = window.Date.RealDate;
  } else {
    RealDate = window.Date;
  }

  // If we're fixing the date we'll no doubt be using it,
  //    so we may as well cache it.
  let fixedDate = new RealDate(dateString);
  if (isNaN(+fixedDate)) { throw Error('Provided date string is invalid. `window.Date` was not modified.'); }

  // The `Date` object is quite complex so let's just copy everything across
  Object.assign(
    FakeDate,
    RealDate,
    {
      RealDate,
      isFake: true,
      // `now` is the only static function that we'll need to modify
      now: fixedDate.getTime.bind(fixedDate),
      revertToDynamicDate() {
        window.Date = RealDate;
      },
    },
  );
  window.Date = FakeDate;

  function FakeDate(...args) {
    // We need to keep the functionality where
    //  if `Date` is called without `new` it returns a string.
    if (new.target) {
      if (args.length) {
        return new RealDate(...args);
      } else {
        return fixedDate;
      }
    } else {
      if (args.length) {
        return RealDate(...args);
      } else {
        return fixedDate.toString();
      }
    }
  }
}