获取当前日期时间的代码如下：

NSDate *dateToDay = [NSDate date];    NSDateFormatter *df = [[NSDateFormatter alloc] init];    [df setDateFormat:@"yyyy-MM-DD HH:mm:ss"];    NSLocale *local = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];    [df setLocale:local];    NSString *myDataString = @"2009-09-15 18:30:00";

从字符串生成日期对象的代码如下：

NSDate *myData = [df dateFromString:myDataString];

日期比较的代码如下：

switch ([dateToDay compare:myData]) {        case NSOrderedSame:            NSLog(@"These dates are the same!");            break;                    case NSOrderedAscending:            NSLog(@"dateToDay is earlier than myDate!");            break;                    case NSOrderedDescending:            NSLog(@"mydate is earlier than dateToDay");            break;                    default:            NSLog(@"Bad times. Invalid enum value returned.");            break;    }

注意：掌握NSDate和NSString相互之间的转换。

完整代码如下：

#import 
     
      int main(int argc, const char * argv[]){        NSDate *dateToDay = [NSDate date];    NSDateFormatter *df = [[NSDateFormatter alloc] init];    [df setDateFormat:@"yyyy-MM-DD HH:mm:ss"];    NSLocale *local = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US"];    [df setLocale:local];    NSString *myDataString = @"2009-09-15 18:30:00";        NSDate *myData = [df dateFromString:myDataString];        switch ([dateToDay compare:myData]) {        case NSOrderedSame:            NSLog(@"These dates are the same!");            break;                    case NSOrderedAscending:            NSLog(@"dateToDay is earlier than myDate!");            break;                    case NSOrderedDescending:            NSLog(@"mydate is earlier than dateToDay");            break;                    default:            NSLog(@"Bad times. Invalid enum value returned.");            break;    }    return 0;}
     

 

 

NSData转换为NSString的代码如下：

NSMutableData *data;    NSString *tmpdata = [[NSString alloc] initWithString:data encoding:NSASCIIStringEncoding];    NSLog(@"[***] DATA:%@" , tmpdata);    [tmpdata release];

NSString转换为NSData的代码如下：

NSString *str = @"teststring";    NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];