그냥 위 소스 무시하고 작성하면
<script>
var today = new Date();
var year = today.getFullYear();
var month = today.getMonth();
var day = today.getDay();
var nextDay = new Date(year, month, day + 30);
var nYear = nextDay.getFullYear();
var nMonth = nextDay.getMonth() + 1;
var nDay = nextDay.getDay();
month++;
month = (month < 10) ? "0" + month : month;
day = (day < 10) ? "0" + day : day;
nMonth = (nMonth < 10) ? "0" + nMonth : nMonth;
nDay = (nDay < 10) ? "0" + nDay : nDay;
alert(year + String(month) + day + "\r\n" + nYear + String(nMonth) + nDay);
today = year + String(month) + day;
nextDay = nYear + String(nMonth) + nDay;
if(dday >= today && dday <= nextDay) {
this.style.backgroundColor = "orange";
}
</script>