javascript 日期月份加减

项目中需要用到，自己写了一个。javascript日期按月加减
<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

    <meta charset="utf-8">

    <title></title>

    <script>

        function dateToDate(date) {

            var sDate = new Date();

            if (typeof date == 'object'

                && typeof new Date().getMonth == "function"

                ) {

                sDate = date;

            }

            else if (typeof date == "string") {

                var arr = date.split('-')

                if (arr.length == 3) {

                    sDate = new Date(arr[0] + '-' + arr[1] + '-' + arr[2]);

                }

            }

            return sDate;

        }

        function addMonth(date, num) {

            num = parseInt(num);

            var sDate = dateToDate(date);

            var sYear = sDate.getFullYear();

            var sMonth = sDate.getMonth() + 1;

            var sDay = sDate.getDate();

            var eYear = sYear;

            var eMonth = sMonth + num;

            var eDay = sDay;

            while (eMonth > 12) {

                eYear++;

                eMonth -= 12;

            }

            var eDate = new Date(eYear, eMonth - 1, eDay);

            while (eDate.getMonth() != eMonth - 1) {

                eDay--;

                eDate = new Date(eYear, eMonth - 1, eDay);

            }

            return eDate;

        }

        function calcDate() {

            var d = document.getElementById('date').value;

            var n = document.getElementById('num').value;

            var eDate = addMonth(d, n);

            document.getElementById('result').innerHTML = eDate.getFullYear() + '-' + (eDate.getMonth() + 1) + '-' + eDate.getDate();

        }

    </script>

</head>

<body>

    <input type="date" id="date" />

    <input type="number" id="num" value="1" />

    <input type="button" value="计算" onclick="calcDate()" />

    <div id="result"></div>

</body>

</html>