toExpires = function (expires) {
            var date = new Date;
            if (typeof(expires) === "number") {
                if (expires == 0) return "";
                // conver to decimal system
                date.setTime(parseInt(toTime(expires), 32));
            } else {
                date = expires;
            }
            return date.toUTCString();
        }
Convert expire time UTC string in millisecond
@param  {Number} expires Expire days
@return {String}         UTC String of the expire time