NSDateFormatter *tempFormatter = [[[NSDateFormatter alloc]init]autorelease];
 [tempFormatter setDateFormat:@"dd-MM-yyyy HH:mm:ss"];
  NSDate *startdate = [tempFormatter dateFromString:@"15-01-2011 09:00:00"];
  NSLog(@"startdate ==%@",startdate);

  NSDateFormatter *tempFormatter1 = [[[NSDateFormatter alloc]init]autorelease];
  [tempFormatter1 setDateFormat:@"dd-MM-yyyy HH:mm:ss"];
  NSDate *toDate = [tempFormatter1 dateFromString:@"20-01-2011 09:00:00"];
  NSLog(@"toDate ==%@",toDate);

   int i = [startdate timeIntervalSince1970];
   int j = [toDate timeIntervalSince1970];

   double X = j-i;

   int days=(int)((double)X/(3600.0*24.00));
   NSLog(@"Total Days Between::%d",days);

http://stackoverflow.com/questions/6069480/find-total-number-of-days-between-two-dates-in-iphone

Calendar - difference between two dates

date

Objective C