example.js

Date.fake(new Date(2019, 0 , 1))

var a = new Date(); // Tue Jan 01 2019 00:00:00 GMT-0300 (ART)

Date.unfake();

a.getTime() === new Date(2019, 0, 1).getTime() //true

fakeDate.js

var oldDate = global.Date;

global.Date.fake = function (fix) {
  var time = fix.getTime();  
  
  global.Date = function () {
    return new oldDate(time);
  };
  
  global.Date.prototype = Object.create(oldDate.prototype);
  global.Date.prototype.constructor = global.Date;

  global.Date.unfake = function () {
    global.Date = oldDate; 
  };
};