[iOS]NSDate 시간 차이 구하기

NSDate *pastDate = ...;
NSDate *nowDate = [NSDate date];

NSCalendar *calendar = [NSCalendar currentCalendar];
NSUInteger calendarUnit = NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;

NSDateComponents *dateComp = [calendar components: calendarUnit
                                fromDate: nowDate
                                  toDate: pastDate
                                 options:0];
NSInteger day = dateComp.day;
NSInteger hour = dateComp.hour;
NSInteger min = dateComp.minute;
NSInteger sec = dateComp.second;