function Timer(startTime, endTime) {
    this.startDate = new Date(startTime);
    this.endDate = new Date(endTime);
    this.startTime = this.startDate.getTime();
    this.endTime = this.endDate.getTime() - this.startTime;
    this.countDownTime = this.endTime;
}


Timer.prototype.start = function () {
    this.countDown();
};


Timer.prototype.countDown = function () {
    this.countDownTime -= 1000;
    var time = new Date(this.countDownTime), that = this;
    
    $('.hour').text(time.getUTCHours());
    $('.minute').text(time.getUTCMinutes());
    $('.second').text(time.getUTCSeconds());
    
    this.timer = setTimeout(function () {
        that.countDown.call(that);
    }, 1000);
    if (this.countDownTime <= 0) {
        clearTimeout(this.timer);
    }
};


var a = new Timer('2015-04-21 16:43:20', '2015-04-21 16:43:22');
a.start();