NSCalendarUnitSecond returning zero - ios

Question

I have this method returning a string. But the seconds value is always zero. What am I doing wrong?
-(NSString*)secondsBetweenDate:(NSDate*)startDate andDate:(NSDate*)endDate {
NSCalendar *calendar = [NSCalendar currentCalendar];
unsigned int unitFlags = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSDateComponents *difference = [calendar components:unitFlags fromDate:startDate toDate:endDate options:0];
long hour = [difference hour];
long min = [difference minute];
long sec = [difference second];
NSLog(#"Hour: %ld Min: %ld Sec: %ld", hour, min, sec);
return [NSString stringWithFormat:#"%02ld:%02ld:%02ld", hour, min, sec];
}

Answer 1

Why don't you use the default NSDate difference calculations?
Returns an NSTimeInterval which is in seconds:
typedef double NSTimeInterval; Description Used to specify a time
interval, in seconds.
Operation is:
[aDate timeIntervalSinceDate:anotherDate];

Answer 2

Your code is correct. If you use the dates that actually differ in their seconds value it yields the expected results.
[self secondsBetweenDate:[NSDate dateWithTimeIntervalSinceNow:-99999] andDate:[NSDate date]];
logs:
Hour: 27 Min: 46 Sec: 39
And 99999 Seconds is 27*60*60 +46*60 +39
In our discussion in the comments we discovered that you had more of a conceptional problem. If you need to show a countdown or stopwatch type string you have to use the current time as one of the date parameters.
So if you want to show the time that has passed since a specific date (like a stop watch) you use:
NSDate *now = [NSDate date];
string = [self secondsBetweenDate:yourStartDate andDate:now];
If you want to show the time until a specific date (like a countdown) you use:
NSDate *now = [NSDate date];
string = [self secondsBetweenDate:now andDate:yourEndDate];
Btw: If you are only targetting iOS 8 and later you can use NSDateComponentsFormatter to format your date
NSDateComponentsFormatter *df = [[NSDateComponentsFormatter alloc] init];
df.unitsStyle = NSDateComponentsFormatterUnitsStylePositional;
df.allowedUnits = NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
NSString *dateString = [df stringFromDate:startDate toDate:[NSDate date]];