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copied!<p>I know this is old question, but I want to share my way to achieve this. We have method (timeIntervalSinceDate:) to calculate the interval and have fixed amount of seconds to count down. In my case 300 s.</p> <p><strong>UPDATE JULY, 2015 SWIFT</strong></p> <pre><code>@IBAction func startTimer(sender: UIButton) { sender.selected = !sender.selected; //if selected fire timer, otherwise stop if (sender.selected) { self.timer = NSTimer.scheduledTimerWithTimeInterval(0.1, target: self, selector: Selector("updateTimer"), userInfo: nil, repeats: true); self.startDate = NSDate(); } else { self.stopTimer(); } } func stopTimer() { self.timer.invalidate(); } func updateTimer() { // Create date from the elapsed time var currentDate:NSDate = NSDate(); var timeInterval:NSTimeInterval = currentDate.timeIntervalSinceDate(self.startDate!); //300 seconds count down var timeIntervalCountDown = 300 - timeInterval; var timerDate:NSDate = NSDate(timeIntervalSince1970: timeIntervalCountDown); // Create a date formatter var dateFormatter = NSDateFormatter(); dateFormatter.dateFormat = "mm:ss"; dateFormatter.timeZone = NSTimeZone(forSecondsFromGMT: 0); // Format the elapsed time and set it to the label var timeString = dateFormatter.stringFromDate(timerDate); self.timerLabel?.text = timeString; } </code></pre> <p><a href="https://github.com/gneil90/countdown-label" rel="nofollow noreferrer">Working github swift sample project</a></p> <p><strong>Objective-C</strong></p> <pre><code>- (void)updateTimer { // Create date from the elapsed time NSDate *currentDate = [NSDate date]; NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:self.startDate]; NSLog(@"time interval %f",timeInterval); //300 seconds count down NSTimeInterval timeIntervalCountDown = 300 - timeInterval; NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeIntervalCountDown]; // Create a date formatter NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; [dateFormatter setDateFormat:@"mm:ss"]; [dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]]; // Format the elapsed time and set it to the label NSString *timeString = [dateFormatter stringFromDate:timerDate]; self.stopWatch.text = timeString; } - (void)stopTimer { [self.stopWatchTimer invalidate]; self.stopWatchTimer = nil; [self updateTimer]; } - (void)startTimer { if (self.stopWatchTimer) { [self.stopWatchTimer invalidate]; self.stopWatchTimer = nil; } self.startDate = [NSDate date]; // Create the stop watch timer that fires every 100 ms self.stopWatchTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/10.0 target:self selector:@selector(updateTimer) userInfo:nil repeats:YES]; } </code></pre> <p><strong>UPDATE:</strong></p> <p>it is extremely expensive to create a NSDateFormatter each time you pass through updateTimer. Far better to create the NSDateFormatter outside the loop and reuse it. Credits: @siburb</p>

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