iPhone sdk: Countdown timer

Please help. I want to create countdown timer using iphone sdk. I have current time:

NSDate *today = [NSDate date];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"HH:mm"];
NSString *currentTime = [dateFormatter stringFromDate:today];
[dateFormatter release];

and time(s) from database. Eg.: currentTime is 19:18:30, time from database is 19:2开发者_开发百科0:01. So the timer should be set to 0:01:31 and begin the countdown. When reach the 0:00:00, take another time from database (eg: 19:21:00), set up timer (0:00:59) and star countdown again. When reach the 0:00:00, take... The timer should be displayed by UILabel.

Please help. It begins to make me mad. :)

try this. I've just written only required methods for the solution. Don't forget to implement dealloc etc.

CountdownViewController.h

  @interface CountdownViewController : UIViewController {     
        IBOutlet UILabel *clockLabel;
        NSDate *databaseDate;
        NSTimer *timer;
    }

    @property(nonatomic, retain) IBOutlet UILabel *clockLabel;
    @property(nonatomic, retain) NSDate *databaseDate;

    @end

CountdownViewController.m

@implementation CountdownViewController

@synthesize clockLabel;
@synthesize databaseDate;

- (void) showClock {
    NSTimeInterval remainingSec = [databaseDate timeIntervalSinceNow];
    if (!timer || remainingSec <= 0) {
        [timer invalidate];
        timer = nil;
        // getting time from database
        self.databaseDate = [NSDate dateWithTimeIntervalSinceNow:20.0];
        remainingSec = [databaseDate timeIntervalSinceNow];
        timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self 
                                               selector:@selector(showClock)  
                                               userInfo:nil 
                                                repeats:YES];
    }

    NSInteger hours = remainingSec / 3600;
    NSInteger remainder = ((NSInteger)remainingSec)% 3600;
    NSInteger minutes = remainder / 60;
    NSInteger seconds = remainder % 60;

    clockLabel.text = [NSString stringWithFormat:@"%02d:%02d:%02d", hours, minutes, seconds];
}

@end