You cannot mix data types within a double matrix, but you can mix data types within a table.
So, you cannot convert two columns of A into datetime before creating the table.
I'm curious why you cannot create your table empty and then add colums as needed.
However, if that doesn't work for some reason, maybe this is what you are looking for?
tbl.A2 = NaT(height(A),1);
tbl.A6 = NaT(height(A),1)