js取日期的前一天

/****************************************
 * 取前一天的日期
 * 参数格式:2014-06-02
 * 结果格式:2014-06-01
 ***************************************/
function convertDate(beginTime)
{
	var now = new Date(beginTime);  
	var years = now.getFullYear();  
	var months = now.getMonth()+1;  
	var days = now.getDate(); 
	
	var NextNow = addDate(years+"-"+months+"-"+days,-1);  
	years = NextNow.getFullYear();  
	months = NextNow.getMonth()+1;  
	days = NextNow.getDate();  
	if((months+"").length==1)
	{
		months = "0" + months;
	}
	if((days+"").length==1)
	{
		days = "0" + days;
	}
	return years+"-"+months+"-"+days;
}
/*****************************************
 * 根据 日期,偏移量 获取新的日期
 * 参数dd的格式:2014-06-01
 ****************************************/
function addDate(dd,dadd)
{  
	var arr = dd.split("-");
	var timestr = "";
	for(var i=0;i<arr.length;i++)
	{
	    if(timestr.length>0)
	    {
	       timestr = timestr + "-";
	    }
	    if(arr[i].length==2 || arr[i].length==4)
	    {
	       timestr = timestr + arr[i];
	    }
	    if(arr[i].length==1)
	    {
	       timestr = timestr + "0" + arr[i];
	    }
	}
	var a = new Date(timestr);  
	a = a.valueOf();  
	a = a + dadd * 24 * 60 * 60 * 1000;  
	a = new Date(a);  

	return a;  
}