iOS－字符串转化成NSDate类型 计算与当前时间的相差 月数 天数

NSString *dateStr=[dicobjectForKey:@"date"];// 2012-05-17 11:23:23

   NSCalendar *gregorian = [[NSCalendaralloc]initWithCalendarIdentifier:NSGregorianCalendar];  

    NSUInteger unitFlags =NSMonthCalendarUnit |NSDayCalendarUnit;  

   NSDateFormatter *format=[[NSDateFormatteralloc]init];

    [formatsetDateFormat:@"yyyy-MM-dd HH:mm:ss"];

    NSDate *fromdate=[format dateFromString:dateStr];

    NSTimeZone *fromzone = [NSTimeZone systemTimeZone];

    NSInteger frominterval = [fromzone secondsFromGMTForDate: fromdate];

    NSDate *fromDate = [fromdate  dateByAddingTimeInterval: frominterval]; 

     NSLog(@"fromdate=%@",fromDate);

    [format release];

    NSDate *date = [NSDate date];

   NSTimeZone *zone = [NSTimeZonesystemTimeZone];

    NSInteger interval = [zone secondsFromGMTForDate: date];

    NSDate *localeDate = [date  dateByAddingTimeInterval: interval]; 

    NSLog(@"enddate=%@",localeDate);

    NSDateComponents *components = [gregorian components:unitFlagsfromDate:fromDatetoDate:localeDate options:0];  

    NSInteger months = [components month];  

    NSInteger days = [components day];//年[components year]

    NSLog(@"month=%d",months);

    NSLog(@"days=%d",days);

    [gregorian release];

    if (months==0&&days==0) {

        dateStr=[[dateStrsubstringFromIndex:11]substringToIndex:5];

        cell.textLabel.text=[NSStringstringWithFormat:@"今天 %@",dateStr];//今天 11:23

    }elseif(months==0&&days==1){

        dateStr=[[dateStrsubstringFromIndex:11]substringToIndex:5];

        cell.textLabel.text=[NSStringstringWithFormat:@"昨天 %@",dateStr];//昨天 11:23

    }else{

        dateStr=[dateStrsubstringToIndex:10];

        cell.textLabel.text=dateStr;

    }