Get previous value of an observable in subscribe of same observable

 self.year = ko.observable().extend({ number: true });
    self.holdoldValueforYear=ko.observable();



    self.year.subscribe(function (previousValue) {
        //I'd like to get the previous value of 'year' here before it's set to newValue
        self.holdoldValueforYear(previousValue);
    }, self, "beforeChange");


    self.year.subscribe(function (yearValue) {
        if (yearValue < self.holdoldValueforYear()) {
            self.year(self.holdoldValueforYear());
        }
    },self);