iOS 得到当前时间之前和之后N天的日期

   dis = 3; //前后的天数
    
    NSDate*nowDate = [NSDate date];
    NSDate* theDate;
    
    if(dis!=0)
    {
        NSTimeInterval  oneDay = 24*60*60*1;  //1天的长度

        theDate = [nowDate initWithTimeIntervalSinceNow: +oneDay*dis ]; 
        //or
theDate = [nowDate initWithTimeIntervalSinceNow: -oneDay*dis ]; 
    }
    else
    {
        theDate = nowDate;
    }

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关键就是这句了：theDate = [nowDate initWithTimeIntervalSinceNow:+oneDay*dis ];