this is one of the few scenarios where it makes sense to have separate year, month, and day columns -- use SMALLINT, TINYINT, TINYINT, and let them go NULL if you do not know a value
if you were to use a single column, then in order for it to carry a value which includes 0's or X's or blanks, it would have to be a character column (VARCHAR, or, in Access, TEXT), and then it would be extremely difficult to search on parts of the value
hence three integer nullable columns