How to add a private function in a backbone.js model?

Is there a way to make a function private in backbone, so that it is only exposed to the model itself and also have access to this ?

How can I make updateTime private?

   var Timeline = Backbone.Model.extend({
        url: 'servertime/',

        start: function(){
            this.fetch({
                success: function(timeline, response){
                    timeline.updateTime();
                    setInterval(function() {
                        timeline.updateTime();
                    }, 60 * 1000);
                }
            });
        },

        updateTime: function(){  //How can I make this private?
            this.time = ...
        }
    });