JavaScript获取距离某天前或后的日期

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/** 
* 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; 
}