trunc('01-02-2009') ONLY works because your NLS_DATE format is mm-dd-yyyy. At my site it is dd-mon-rr. Do not get into the habit of relying on the NLS settings, if it changes or the software is at another site it will always fail. Use

to_date('01-02-2009','mm-dd-yyyy')

Because the trunc is expecting a date, it implicitly converts the string to a date and then truncs it. A very bad habit to get into.