OC笔记NSDate

```python

//-----1--NSDate -------

    //NSDate的定义
    NSDate * nowDate = [NSDate date];
    NSLog(@"%@",nowDate);
    
    //NSDateFormatter的定义
    NSDateFormatter *df = [[NSDateFormatter alloc]init];
    
    //日期的加减运算
    nowDate = [nowDate dateByAddingTimeInterval:-5 * 60 * 60];
    NSLog(@"%@",nowDate);
    [df setDateFormat:@"dd/MM yyyy HH:mm:ss"];
    NSString *dateStr = [df stringFromDate:nowDate];
    NSLog(@"%@",dateStr);
```