-(ContentViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController{
    ContentViewController *controller = (ContentViewController *)viewController;
    //与今天的比较,并且只在向后翻中用到
    if([controller.specialDay isEqualToDate:[NSDate date]]){
        return nil;
    }else{
        ContentViewController *newController =               
        [[ContentViewController alloc]init];
        newController.specialDay = [NSDate dateWithTimeInterval:24*60*60 sinceDate:controller.specialDay];
        return newController;
    }
}

//执行起来后,虽然默认的日期是今天,但是第一条语句似乎不起作用,依然可以向后翻页