NSDate returns a negative NSTimeInterval

I have the following code:

- (void)pointsStartedDisplaying {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        dispatch_async(dispatch_get_main_queue(), ^{
            self.timeStartedDisplayingPoint = [NSDate date];
            self.stopButton.enabled = YES;

            [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(updateElapsedTime:) userInfo:nil repeats:YES];

- (void)updateElapsedTime:(NSTimer *)timer {
    NSTimeInterval elapsedTimeDisplayingPoints = [self.timeStartedDisplayingPoint timeIntervalSinceDate:[NSDate date]];
    self.elapsedTimeValueLabel.text = [NSString stringWithFormat:@"%f seconds", elapsedTimeDisplayingPoints];

However, the value of elapsedTimeDisplayingPoints is perfectly accurate, except negative each time. Why does it return -3, -4, -5, etc?


If the receiver is earlier than anotherDate, the return value is negative. Which is the case with your example. You can use fabs() to get a positive value.

You get negative numbers because self.timeStartedDisplayingPoint is before [NSDate date].

You need:

NSDate *now = [NSDate date];
NSTimeInterval elapsedTimeDisplayingPoints = [now timeIntervalSinceDate:self.timeStartedDisplayingPoint];

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