获取日期：昨天，前台，明天，后天,获取前几天或后几天

1.参数
 num：该参数为数字，为正数就是当前日期天数往后移的天数，比如正数1就是明天，-1就是昨天
 str:日期中间的间隔符，输入'-'就为'YYYY-MM-DD',或输入'/'就为'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;
        }