iOS 得到当前时间之前和之后N天的日期

http://blog.sina.com.cn/s/blog_6f72ff900102v2vq.html

NSInteger 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;
}