why am i getting negative values for my timeinterval? - iphone

Question

I have no idea what is happening. I am trying to make a timer that works by comparing dates. When I start the timer, sometimes it works and then randomly stops, and sometimes it just returns the value of the timeInterval. It seems to only work correctly when the time interval is negative. This is my method:
-(IBAction)startTimer:(id)sender{
if (timer == nil) {
[startButton setTitle:#"Start" forState:UIControlStateNormal];
timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:#selector(timerAction:) userInfo:nil repeats:YES];
date = [NSDate dateWithTimeIntervalSinceNow:testTask.timeInterval]; //instance variable
} else {
[startButton setTitle:#"Stop" forState:UIControlStateNormal];
[timer invalidate];
timer = nil;
}
}
-(void)timerAction:(NSTimer *)t
{
NSDate *currentDate = [NSDate date];
NSTimeInterval updatedTimeInterval = [date timeIntervalSinceDate:currentDate];
if (updatedTimeInterval > 0){
if (self.timer)
{
[self timerExpired];
[self.timer invalidate];
self.timer = nil;
}
}
else
{
testTask.timeInterval = updatedTimeInterval;
NSLog(#"%.2f", testTask.timeInterval);
NSError *error;
if (![self.context save:&error]) {
NSLog(#"couldn't save: %#", [error localizedDescription]);
}
}
NSUInteger seconds = (NSUInteger)round(testTask.timeInterval);
NSString *string = [NSString stringWithFormat:#"%02u:%02u:%02u",
seconds / 3600, (seconds / 60) % 60, seconds % 60];
timerLabel.text = string;
NSLog(#"%f", testTask.timeInterval);
}

Answer 1

You're subtracting the later date, currentDate, from the earlier date. I told you to do it the other way around: [[NSDate date] timeIntervalSinceDate:date]

Answer 2

There's a reason timeIntervalSinceNow is not called timeIntervalUntilNow...
Also known as: the time elapsed since now is negative if the date in question is earlier than the current date/time. It would be positive if the date was later in time than the current date. (Just a bit of logic and/or English semantics :))
Earlier date Current date
^ ^
+------------------+
since now: (earlier date) - (current date)... that's < 0