hi guys , this is my first ipost 
In objective c, many times u need to use countdown in your iapps so simple code for same
first declare Nstimer object and one UILabel (to show count down timing on app )in .h file
@interface numberSeqViewController : UIViewController
{
NSTimer * countDown;
UILabel* Timerlbl;
}
@property(nonatomic,retain) NSTimer *countDown;
@property(nonatomic,retain) UILabel*Timerlbl;
in .m file first synthesize both objects
@synthesize countDown,Timerlbl;
int secondsLeft=5;//just to calculate remaining seconds
i assume you have added Label programatically or linked with in IB
initialize NStimer object
countDown=[NSTimer scheduledTimerWithTimeInterval:5.0 target:self selector:@selector(TimeOver) userInfo:nil repeats:YES];
scheduledTimerWithTimeInterval- time interval in seconds
selector – action method name
Repeat-YES/NO
-(void)TimeOver
{
secondsLeft–;
int seconds = (secondsLeft %3600) % 60;
Timerlbl.text = [NSString stringWithFormat:@"%02d",seconds];
if (seconds==0)
{
UIAlertView *pAlert = [[UIAlertView alloc]initWithTitle:@”Sorry!!” message:@”Time Over” delegate:self cancelButtonTitle:@”OK” otherButtonTitles:@”Cancel”,nil];
[pAlert show];
[pAlert release];
}
}
thats it now run the app .you will see that label is displayed and changing its text as count down….
thats cool ..
ok ,now you need to stop timer as per your requirement
if (countDown!=nil)
{
[countDown invalidate];
countDown=nil;
}
call inValidate method of NSTimer to stop it and set it to nil
tip: remeber you only invalidate the timer which is already inittiated ,you cannot invalidate if it is already nil,it will heads to Exception
thats it ..Njoy icoding 
