This gives you a double which is the current number of seconds since Jan 1st 2001. It yields sub-millisecond precision over a range of 10,000 years.
    double howmanysecondselapsed = [NSDate timeIntervalSinceReferenceDate];
    NSLog(@"secs: %f",howmanysecondselapsed);
Example output:

secs: 249890891.253101

It is the same precision on OS X and the iPhone.