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;
    }

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