分开来写，让你看的清楚些
<script>
d = new Date();
y = d.getYear();
m = d.getMonth() + 1;t = new Date(y, m, 0);
alert(t.getDay());
</script>
<script>
d = new Date();
y = d.getYear();
m = d.getMonth() + 1;t = new Date(y, m, 0);
alert(t.getDay());
</script>
y = d.getYear();
m = d.getMonth() + 1;t = new Date(y, m, 0);
alert(t.getDate());
http://community.csdn.net//Expert/TopicView2.asp?id=4360247&datebasetype=now
js FAQ贴
http://community.csdn.net//Expert/TopicView2.asp?id=4356360&datebasetype=now
DHTML手册,js手册,dom手册下载
http://community.csdn.net//Expert/TopicView2.asp?id=4356325&datebasetype=now