Not a valid month error

I have a table with one of the columns as DDate(char(6) as the data type). I am trying to use to_date function in SELECT query to display all the records but it's giving "Not a valid month" for all the dates after the year 89. I tries to fix it with whatever infomration I could get from the Interenet, but I am unable to correct the error.Any ideas as to where the problem is? Received on Wed Mar 30 2005 - 08:17:35 CST