JavaScript ajouter des heures à l'heure

J'ai ce code JavaScript qui devrait montrer l'heure. Ça marche. Je ne pourrais pas pouvoir ajouter du temps supplémentaire cependant. Disons que je veux ajouter 1 heure.

        <script type="text/javascript">
        Date.prototype.addHours = function(h) {    
           this.setTime(this.getTime() + (h*60*60*1000)); 
           return this;   
        }
        // This function gets the current time and injects it into the DOM

        function updateClock() {
            // Gets the current time
            var now = new Date();

            // Get the hours, minutes and seconds from the current time
            var hours = now.getHours();
            var minutes = now.getMinutes();
            var seconds = now.getSeconds();

            // Format hours, minutes and seconds
            if (hours < 10) {
                hours = "0" + hours;
            }
            if (minutes < 10) {
                minutes = "0" + minutes;
            }
            if (seconds < 10) {
                seconds = "0" + seconds;
            }

            // Gets the element we want to inject the clock into
            var elem = document.getElementById('clock');

            // Sets the elements inner HTML value to our clock data
            elem.innerHTML = hours + ':' + minutes + ':' + seconds;
        }
    function start(){
        setInterval('updateClock()', 200);
    }
    </script>