Immutable Moments

moment-immutable.js

(function (root, factory) {
	"use strict";
	if (typeof define === 'function' && define.amd) {
		define(['moment'], factory);
	} else if (typeof exports === 'object') {
		module.exports = factory(require('moment'));
	} else {
		factory(root.moment);
	}
}(this, function (moment) {
  "use strict";
  function wrap (name) {
    var old = moment.fn[name];
    moment.fn[name] = function () {
      return old.apply(this.clone(), arguments);
    };
  }
  var methods = 'add subtract startOf endOf'.split(' ');
  for (var i = 0; i < methods.length; i++) {
    wrap(methods[i]);
  }
}));
  

An example here. http://jsfiddle.net/timrwood/tdkfqkdg/

Javascript is magic.