Simple TimeSpan for javascript

TimeSpan.js

    function TimeSpan(seed) {
        //"00:00:00"
        var self = this;
        self.hour = seed.substr(0, 2);
        self.minute = seed.substr(3, 2);
        self.second = seed.substr(6, 2);

        self.addSecond = function () {
            self.second++;
            if (self.second >= 60) {
                self.second = 0;
                self.minute++;
            }

            if (self.minute >= 60) {
                self.minute = 0;
                self.hour++;
            }
        };

        self.serialize = function () {
            return ("0" + self.hour).slice(-2) + ":" + ("0" + self.minute).slice(-2) + ":" + ("0" + self.second).slice(-2);
        };
    }