在IOS中如何检测日期

写了一个应用：

UIAlertView *alert = [[UIAlertView alloc]

                      initWithTitle:@"Icon Contest!"
                      message:@"Design a 1024 x 1024 pixels icon for Example. The best one will be on the app's icon! Send them to: example@example.com."
                      delegate:self
                      cancelButtonTitle:@"More Info"
                      otherButtonTitles:@"Yes",@"No", nil];

[alert show];

其中有最后期限，然后在一月31号结束时应用显示不同的警报。