Getting future date using NSDate and NSTimeInterval - ios

Question

I'm creating a simple timer but am failing to get the resulting date that I'm after.
My problem is that I'm getting -34481.274523 from my NSTimeInterval (nslog at the end) whereas I'm wanting the number of seconds since the current date (currentDate). stopDate is always a date before currentDate.
The code snippet below is called from an nstimer every second. Rather than simply increment an int I'm using a date as it's easier to perform other date related calculations on and format for display.
Here is my code:
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:stopDate];
NSDate *timerDate = [NSDate dateWithTimeIntervalSince1970:timeInterval];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"mm:ss"];
NSString *timeString=[dateFormatter stringFromDate:timerDate];
NSLog(#"nstimeinterval: %f", timeInterval);
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components: ( NSHourCalendarUnit | NSYearCalendarUnit) fromDate:timerDate];
NSLog(#"year: %li", (long)components.year);
if (components.year == 2000) {
timeString = #"00:00";
return;
}
NSLog(#"timer: %#", timeString);
timerTicksForCounter = timeString;
NSLog(#"hour: %li", (long)components.hour);
//Used to update uilabel
[self updateTimerLabel];
if (components.year < 2000) {
return;
}
if (components.hour == 0) {
..... I'm doing a bunch of stuff here