What error message do you get?
What database are you connecting to via odbc?
If it's ms access, for instance, it likes dates wrapped in "#"
odbc_do($db,"INSERT INTO Classified (ClassAuthor,ClassMail,ClassLocation,ClassCost,ClassBody,ClassDate) VALUES ('$postauthor','$postmail','$postlocation','$postcost','$postbody',#now()#)");