Do you mean calendar days or 24-hour periods? i.e. is Tuesday at 9PM a day before Wednesday at 6AM, or less than one day?
If you mean the former, it's a bit complicated and you'll have to resort to manipulations via NSCalendar and NSDateComponent which I don't recall off the top of my head.
If you mean the latter, just get the dates' time intervals since the reference date, subtract one from the other, and divide by 24 hours (24 * 60 * 60) to get the approximate interval, leap seconds not included.
