关于NSString显示NSDate为000

使用NSDate将日期保存到NSString中，但是日期变为000.000。

NSDate* date = [NSDate date];
 NSDateFormatter* formatter = [[NSDateFormatter alloc] init];
 [formatter setDateFormat:@"yyyy-MM-dd HH:MM:SS"];

NSString* str = [formatter stringFromDate:date];
 NSString*content_AddedTime=str;

NSString *post =[[NSString alloc] init];
 post = [NSString stringWithFormat:@"userId=%@&catalogID=%@&content_AddedTime=%@&organizationCode=%@",userId,catalogID,content_AddedTime,organizationCode];