1.引數
 num：該引數為數字，為正數就是當前日期天數往後移的天數，比如正數1就是明天，-1就是昨天
 str:日期中間的間隔符，輸入'-'就為'YYYY-MM-DD',或輸入'/'就為'YYYY/MM/DD'
function getDay(num, str) {
            var today = new Date();
            var nowTime = today.getTime();
            var ms = 24*3600*1000*num;
            today.setTime(parseInt(nowTime + ms));
            
 
var oYear = today.getFullYear();
            var oMoth = (today.getMonth() + 1).toString();
            if (oMoth.length <= 1) oMoth = '0' + oMoth;
            var oDay = today.getDate().toString();
            if (oDay.length <= 1) oDay = '0' + oDay;
            return oYear + str + oMoth + str + oDay;
        }