.h file
 
// declare timer 
 
NSTimer *countDownTimer;
@property (nonatomic, retain) NSTimer *countDownTimer;
 
.m file
 
// stop the timer before it is used
// this is useful sometimes if not the timer might be run twice
[self.countDownTimer invalidate];
 
// this will create and RUN/SCHEDULE the timer all at the same time.
self.countDownTimer = [NSTimer scheduledTimerWithTimeInterval:2 target:self     selector:@selector(callBackFunctionHere:) userInfo:nil repeats:NO];