NSTimer time increment - objective-c

Question

I'm a beginner in obj-C for iOS platform and am trying to build a few simple project to build my foundation.
I have a button which increase the NSTimer time for the label, but when I use NSLog to log the time, it uses the value before time increment was implemented. I need to be able to log a updated time (after increment), as I require that value and am implementing more function into the IBAction after I solve this portion.
E.g at 15min I press, the NSLog will read it as "00:15:00.0" rather than "00:35:00.0".
- (IBAction)onSkipPressed:(id)sender {
startDate = [startDate dateByAddingTimeInterval:-1200];
NSLog(#"%#",self.timeLabel.text);
}
Any one know the reason for this issue? And how should I solve it such that NSLog will read it as "00:35:00.0" if I invoke this IBAction at 15min.
EDIT - The start button will start the timer and timeLabel will get the string. Sorry for missing out such a important detail. I don't think there are any other code in the project which is related to this functionality already. Thank you for pointing it out to me.
- (void)updateTimer
{
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:startDate];
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"HH:mm:ss.S"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0.0]];
NSString *timeString=[dateFormatter stringFromDate:timerDate];
timeLabel.text = timeString;
}
my IBAction to fire the timer
- (IBAction)onStartPressed:(id)sender {
startDate = [NSDate date];
gameTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/10.0
target:self
selector:#selector(updateTimer)
userInfo:nil
repeats:YES];
//hide start button and show timeLabel
startButton.hidden=true;
timeLabel.hidden=false;
}

Answer 1

I went back to do a few revision with tutorials involving NSTimer. And turns out all I was missing was 1 line [self updateTimer]
- (IBAction)onSkipPressed:(id)sender {
startDate = [startDate dateByAddingTimeInterval:-1200];
[self updateTimer];
NSLog(#"%#",self.timeLabel.text);
}
This solve my issue and the timeLabel.text is updated for me to log the information.

Answer 2

Um, why are you passing in negative 1200?
// this subtracts 1200 seconds from your date, no?
startDate = [startDate dateByAddingTimeInterval:-1200];
Shouldn't you do:
// add 30 minutes (60 seconds a minute x 30 minutes) to your time interval
startDate = [startDate dateByAddingTimeInterval:(60 * 30)];
...
NSLog(#"%#",self.timeLabel.text);
Or am I misunderstanding something?