JavaScript获取距离某天前或后的日期

/**

* param  Date Object：Mon May 11 13:53:08 UTC+0800 2015 

* n 自param 起向后多少天

* return Date Object：Mon May 11 13:53:08 UTC+0800 2015

*/

function getDateBeforeParamDate(param,n){

var result = new Date();

if(param==null){

param = result;

}

if(n==null){

n=0;

}

param = new Date(param);

millSecond = param.valueOf();

millSecond =  millSecond - n*24*3600*1000;

result = new Date(millSecond);

result = getTodayFormatea(result);

return result;

}

/**

* param  Date对象：Mon May 11 13:53:08 UTC+0800 2015 

* n 自param 向前多少天

* return Date Object：Mon May 11 13:53:08 UTC+0800 2015

*/

function getDateNextParamDate(param,n){

var result = new Date();

if(param==null){

param = result;

}

if(n==null){

n=0;

}

param = new Date(param);

millSecond = param.valueOf();

millSecond = millSecond + n*24*3600*1000;

result = new Date(millSecond);

result = getTodayFormatea(result);

return result;

}