CBTimer is an simple Objective C timer object for count downs. You can set a limit and start Ascending or Descending. Using blocks for Callbacks and isFinished boolean. Using CADisplayLink

CBTimer Usage

// Do any additional setup after loading the view.
    [[CBTimer instance] startTimerWithMaxTime:5 mode:CBTimerModeDecrease timerDoneBlock:^ (CFTimeInterval elapsed, BOOL isFinished)
    {
        self.counterLabel.text = [NSString stringWithFormat:@"%.3f",elapsed];
        
        if (isFinished)
        {
            UIAlertView *finished =[[UIAlertView alloc] initWithTitle:@"Finished" message:@"Finito" delegate:nil cancelButtonTitle:@"OKAY" otherButtonTitles:nil];
            [finished show];
        }
    }];

CBTimer.h

//
//  CBTimer.h
//  CBColourBlind
//
//  Created by Daniel Tavares on 03/09/2013.
//  Copyright (c) 2013 Daniel Tavares. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <QuartzCore/QuartzCore.h>

typedef void (^CBTimerTickedBlock)(CFTimeInterval elapsed, BOOL isFinished);


typedef enum
{
    CBTimerModeDecrease,
    CBTimerModeIncrease
} CBTimerMode;

@interface CBTimer : NSObject
{
    CADisplayLink *displayLink;
    CBTimerMode mode;
}

@property (nonatomic, copy) CBTimerTickedBlock doneBlock;

+ (id)instance;

-(void)startTimerWithMaxTime:(double)maxTime
                        mode:(CBTimerMode)mode
              timerDoneBlock:(CBTimerTickedBlock)doneBlock;

@end

CBTimer.m

//
//  CBTimer.m
//  CBColourBlind
//
//  Created by Daniel Tavares on 03/09/2013.
//  Copyright (c) 2013 Daniel Tavares. All rights reserved.
//

#import "CBTimer.h"

static BOOL running;

@implementation CBTimer
{
    CFTimeInterval startTimer;
    CFTimeInterval endTimer;
}

+ (id)instance
{
    static dispatch_once_t pred;
    static CBTimer *instance = nil;
    
    dispatch_once(&pred, ^{ instance = [[self alloc] init]; });
    return instance;
}

-(void)startTimerWithMaxTime:(double)maxTime
                        mode:(CBTimerMode)timerMode
              timerDoneBlock:(CBTimerTickedBlock)doneBlock
{
    
    self.doneBlock = doneBlock; //set block
    startTimer = CFAbsoluteTimeGetCurrent(); //get current time
    endTimer = (CFTimeInterval)maxTime; //set max time
    mode = timerMode;
    
    [displayLink invalidate]; //stop timer just in case
    running = NO;
    
    if(!running)
    {
        displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(startCountDown)];
        [displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
        running = YES;
    }
}

-(void)startCountDown
{
    
    if (running)
    {
        CFTimeInterval diff = 0.0;
        BOOL isFinished = NO;
        
        if (mode==CBTimerModeDecrease)
        {
            diff = endTimer - (CFAbsoluteTimeGetCurrent() - startTimer);
            
            if (diff<=0)
            {
                [displayLink invalidate];
                running = NO;
                
                diff = 0;
                isFinished = YES;
            }
        }
        else
        {
            diff = (CFAbsoluteTimeGetCurrent() - startTimer);
            
            if (diff>=endTimer)
            {
                [displayLink invalidate];
                running = NO;
                isFinished = YES;
                
                diff = endTimer;
            }
        }
       
        //dont let it go below it
        
        if (self.doneBlock)
        {
            self.doneBlock(diff,isFinished);
        }
    }
}

@end