Question about PARTIAL DATE type/s

I have text fields which contains dates in the format 'YYYYMM' (four positions for the year, two for the month).

These fields are contained in text files which I load into the DB.

When I convert this field to a date using 

        to_date("PARTIAL_DATE_FIELD",'YYYYMM'); -- (a cast won't recognize the input as valid)

and so on, what is wrong in this case because nowhere it is said that I have the /first/ of that month, the entry just says that the event in question happened /during/ that month.

Thanks,

RD