js 日期加减

/**
 * 日期加减
 * @param dayNum 可为负数，为负数时，是减去天数。
 * @returns {String}
 */
function addDate(dayNum){	
	var now = new Date();
	var years = now.getFullYear();
	var months = now.getMonth()+1;
	var days = now.getDate();
	var hours = now.getHours();
	
	var a = new Date(months+"/"+days+"/"+years);
	a = a.valueOf();
	a = a + (dayNum) * 24 * 60 * 60 * 1000;
	a = new Date(a);
	var newDate = a;
	years = newDate.getFullYear();
	months = newDate.getMonth()+1;
	days = newDate.getDate();
	var resultDate=years+"-"+months+"-"+days;
	alert("结果日期："+resultDate);
	return resultDate;
	}