XML中NSDateFormatter的空值

     NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
        NSLocale *locale = [[NSLocale alloc] initWithLocaleIdentifier:@"US"];
        [dateFormatter setLocale:locale];
        [dateFormatter setDateFormat:@"MM/dd/yyyy HH:mm:ss ZZZ"];
        currentDownloadedScore.dateEntered = [dateFormatter dateFromString:self.currentValue];
        NSLog(@"Date CV: %@",self.currentValue);
        NSLog(@"Date: %@",[currentDownloadedScore.dateEntered description]);
        [locale release];
        [dateFormatter release];

以下是调试器中的结果:

2010-07-16 08:15:35.741 MyApp[75003:207] Date CV: 07/16/2010 04:21:00 +00:00
2010-07-16 08:15:35.742 MyApp[75003:207] Date: (null)
2010-07-16 08:15:35.745 MyApp[75003:207] Date CV: 07/16/2010 01:09:26 +00:00
2010-07-16 08:15:35.749 MyApp[75003:207] Date: (null)