NSTimeInterval to NSDate

An NSTimeInterval, as its name, um, implies, doesn't represent the same thing as an NSDate. An NSDate is a moment in time. A time interval is a stretch of time. To get a point from an interval, you have to have another point. Your question is like asking "How do I convert 12 inches to a spot on this board I'm cutting?" Well, 12 inches, starting from where?

You need to pick a reference date. This will most likely be the NSDate representing the time that you started your counter. Then you can use +[NSDate dateWithTimeInterval:sinceDate:] or -[NSDate dateByAddingTimeInterval:]

That said, I'm pretty sure you're thinking about this backwards. You're trying to display time elapsed since some starting point, i.e., the interval, not the current time. Every time you update the display, you should just be using the new interval. For example (assuming you have a timer firing periodically to do the update):

- (void) updateElapsedTimeDisplay: (NSTimer *)tim {

    // You could also have stored the start time using
    // CFAbsoluteTimeGetCurrent()
    NSTimeInterval elapsedTime = [startDate timeIntervalSinceNow];

    // Divide the interval by 3600 and keep the quotient and remainder
    div_t h = div(elapsedTime, 3600);
    int hours = h.quot;
    // Divide the remainder by 60; the quotient is minutes, the remainder
    // is seconds.
    div_t m = div(h.rem, 60);
    int minutes = m.quot;
    int seconds = m.rem;

    // If you want to get the individual digits of the units, use div again
    // with a divisor of 10.

    NSLog(@"%d:%d:%d", hours, minutes, seconds);
 }

An easy conversion from and back is shown here:

 NSDate * now = [NSDate date];
 NSTimeInterval  tiNow = [now timeIntervalSinceReferenceDate]; 
 NSDate * newNow = [NSDate dateWithTimeIntervalSinceReferenceDate:tiNow];

Ole K Hornnes

I would advise against using NSDateFormatter if you wish to display time intervals. NSDateFormatter is useful when you wish to display times in the local, or a specific, timezone. But in this case it would be a bug if the time was timezone adjusted (e.g. one day per year has 23 hours).

NSTimeInterval time = ...;
NSString *string = [NSString stringWithFormat:@"%02li:%02li:%02li",
                                              lround(floor(time / 3600.)) % 100,
                                              lround(floor(time / 60.)) % 60,
                                              lround(floor(time)) % 60];

If you have your initial date stored in an NSDate object, you can get a new date any interval in the future. Simply use dateByAddingTimeInterval: like this:

NSDate * originalDate = [NSDate date];
NSTimeInterval interval = 1;
NSDate * futureDate = [originalDate dateByAddingTimeInterval:interval];