您为日期格式使用了错误的格式，也无需创建NSDateFormatter的另一个实例-

You are using wrong format for date format and also there is no need to create another instance of NSDateFormatter -

    NSString *inputString=@"15 February 2012 17:05"; 
    NSDateFormatter *dateFormat = [[NSDateFormatter alloc] init]; 
    [dateFormat setDateFormat:@"dd MMMM yyyy HH:mm"]; 
    NSDate *dateVal = [dateFormat dateFromString:inputString];

    [dateFormat setDateFormat:@"MM/dd/yyyy HH:mm"];
    NSString *outputString = [dateFormat stringFromDate:dateVal];