Time travel feature to be used in tests. This overrides `new Date()` to use the specified date, otherwise will call the original implementation.

time-travel.js

// usage:
//   import { travelTo, travelToPresent } from '../helpers/time-traveling';
//
//   beforeEach() { travelTo(2016, 6, 10); },
//   afterEach()  { travelToPresent(); }
//
const DateOriginal = Date;

export function travelToPresent() {
  window.Date = DateOriginal;
}

export function travelTo(year, month, day, hour, minute, seconds) {
  const TimeTravelingDate = function f() {
    if(this instanceof f) {
      if(arguments.length <= 0) {
        return new DateOriginal(
          year      || 2000,
          month - 1 || 0,
          day       || 1,
          hour      || 0,
          minute    || 0,
          seconds   || 0
        );
      } else {
        let args = [].slice.call(arguments);
        args.unshift(window);
        return new (Function.prototype.bind.apply(DateOriginal, args))();
      }
    } else {
      return DateOriginal.apply(window, arguments);
    }
  };

  TimeTravelingDate.parse     = DateOriginal.parse;
  TimeTravelingDate.UTC       = DateOriginal.UTC;
  TimeTravelingDate.prototype = DateOriginal.prototype;
  window.Date                 = TimeTravelingDate;
}