js获取制定时间往前或往后推多少天

//获取指定日期的相差多少天的日期
function addByTransDate(dateParameter, num) {

var translateDate = "", dateString = "", monthString = "", dayString = "";
translateDate = dateParameter.replace("-", "/").replace("-", "/"); ;

var newDate = new Date(translateDate);
newDate = newDate.valueOf();
newDate = newDate - num * 24 * 60 * 60 * 1000; //备注 如果是往前计算日期则为减号 否则为加号
newDate = new Date(newDate);

//如果月份长度少于2，则前加 0 补位
if ((newDate.getMonth() + 1).toString().length == 1) {
monthString = 0 + "" + (newDate.getMonth() + 1).toString();
} else {
monthString = (newDate.getMonth() + 1).toString();
}

//如果天数长度少于2，则前加 0 补位
if (newDate.getDate().toString().length == 1) {

dayString = 0 + "" + newDate.getDate().toString();
} else {

dayString = newDate.getDate().toString();
}

dateString = newDate.getFullYear() + "-" + monthString + "-" + dayString;
return dateString;

}