Actually depending on the number of tables you may want to do a subselect with unions (each union select does a max on it's own table). Then do a max on the union.
Ex.
SELECT MAX(startdate_d) AS MaxStartState FROM
(
SELECT MAX(startdate_d) AS startdate_d FROM tbl1
UNION
SELECT MAX(startdate_d) AS startdate_d FROM tbl2
UNION
SELECT MAX(startdate_d) AS startdate_d FROM tbl3
) AS MaxedUnionQuery
This should take better advantage of the tables index if one exists on that field then overall max will have a smaller set to work with.
"Don't roll your eyes at me. I will tape them in place." (Teacher on Boston Public)