TimeTravel

var TimeTraveler = (function() {
    var time,
        def = function(value) {
            var values = {},
                timeTraveler = {
                    get value() {
                        var lastTime = Object.keys(values)
                            .filter(function(pastTime) {
                                return pastTime < def.time;
                            })
                            .sort(function(a, b) { return a < b ? - 1 : a === b ? 0 : 1;})
                            .pop();

                        return values[lastTime];
                    },
                    set value(newValue) {
                        values[def.time] = newValue;
                    }
                };

            timeTraveler.value = value;

            return timeTraveler;
        };
    
    Object.defineProperty(def, "time", {
        get : function() {
            return time === undefined ? (new Date()).getTime() : time;
        }, 
        set : function(pTime) {
            time = pTime;
        }
    });
    
    return def;
} ());

var foo = TimeTraveler(200);

setTimeout(function() {
    foo.value = 500; 
    
    TimeTraveler.time = (new Date()).getTime() - 200;
    console.log(foo.value);
}, 500);