SQL Server 2014

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CREATE TABLE table_1 (date_1 DATE);
CREATE TABLE table_2 (date_2 DATE);
INSERT INTO table_1 VALUES ('2013-05-05'), ('2002-02-02'), ('2001-01-01');
3 rows affected

INSERT INTO table_2 VALUES ('2012-05-05'), ('2004-04-02'), ('2006-01-01');
3 rows affected

SELECT * FROM table_1 ORDER BY date_1;

date_1

2001-01-01

2002-02-02

2013-05-05

SELECT * FROM table_2 ORDER BY date_2;

date_2

2004-04-02

2006-01-01

2012-05-05

SELECT MAX(date_1) AS the_date FROM
(
  SELECT date_1 FROM table_1 AS the_date
  UNION
  SELECT date_2 FROM table_2 AS the_date
) AS my_tab;

the_date

2013-05-05