mysql> create table test
    -> (
    ->   a int not null,
    ->   b timestamp not null,
    ->   c int,
    ->   unique key  (a,b)
    -> );
Query OK, 0 rows affected (0.56 sec)

mysql> INSERT INTO test (a,b,c) VALUES (1,timestamp(CURRENT_DATE), 1)
    ->  ON DUPLICATE KEY UPDATE c=VALUES(c);
Query OK, 1 row affected (0.13 sec)

mysql> select * from test;
+---+---------------------+------+
| a | b                   | c    |
+---+---------------------+------+
| 1 | 2014-05-13 00:00:00 |    1 |
+---+---------------------+------+
1 row in set (0.03 sec)

mysql> INSERT INTO test (a,b,c) VALUES (1,timestamp(CURRENT_DATE), 1)
    ->  ON DUPLICATE KEY UPDATE c=VALUES(c);
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test;
+---+---------------------+------+
| a | b                   | c    |
+---+---------------------+------+
| 1 | 2014-05-13 00:00:00 |    1 |
+---+---------------------+------+
1 row in set (0.00 sec)

mysql> INSERT INTO test (a,b,c) VALUES (1,timestamp(CURRENT_DATE), 1)
    ->  ON DUPLICATE KEY UPDATE c=VALUES(c);
Query OK, 0 rows affected (0.00 sec)

mysql> select * from test;
+---+---------------------+------+
| a | b                   | c    |
+---+---------------------+------+
| 1 | 2014-05-13 00:00:00 |    1 |
+---+---------------------+------+
1 row in set (0.00 sec)

mysql>