Create a Timer or Clock with "not 100% precision" based on NSTimers

timer.m

// Create an Outlet for a UIlabel on storyboard and set its text property to it...

- (void)startTimer
{
    if (_timer == nil) {
        _timer = [NSTimer scheduledTimerWithTimeInterval:0.1
                                                  target:self
                                                selector:@selector(timerTick:)
                                                userInfo:nil
                                                 repeats:YES];
    }
}

- (void)stopTimer
{
    [_timer invalidate];
    _timer = nil;
}

- (void)timerTick:(NSTimer *)timer
{
    // Timers are not guaranteed to tick at the nominal rate specified, so this isn't technically accurate.
    // However, this is just an example to demonstrate how to stop some ongoing activity, so we can live with that inaccuracy.
    _ticks += 0.1;
    double seconds = fmod(_ticks, 60.0);
    double minutes = fmod(trunc(_ticks / 60.0), 60.0);
    double hours = trunc(_ticks / 3600.0);
    self.timerLabel.text = [NSString stringWithFormat:@"%02.0f:%02.0f:%04.1f", hours, minutes, seconds];
}