1.首先把String轉為日期：

var str ='2012-08-12 23:13:15';
str = str.replace(/-/g,"/");
var date = new Date(str );

需要注意一點是days引數需要轉int，否則直接會拼接字串

  function addDate(date, days) {
            if (days == undefined || days == '') {
                days = 1;
            }
            var date = new Date(date);
            date.setDate(date.getDate() + parseInt(days)
 
);
            var month = date.getMonth() + 1;
            var day = date.getDate();
            return date.getFullYear() + '-' + getFormatDate(month) + '-' + getFormatDate(day);
        }
        
  // 日期月份/天的顯示，如果是1位數，則在前面加上'0'
        function getFormatDate(arg) {
            if (arg == undefined || arg == '') {
                return '';
            }

            var re = arg + '';
            if (re.length < 2) {
                re = '0' + re;
            }

            return re;
        }