根据两个日期计算相差的年月日

NSCalendar *cal = [NSCalendar currentCalendar];
unsigned int unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit;
NSDateComponents *d = [cal components:unitFlags fromDate:[NSDate date] toDate:newDate options:0];//newDate为未来的某一天
DLog(@"day = %d month = %d year = %d",[d day]+1,[d month],[d year]);