MySQLテーブルの列に現在の日付を自動的に挿入するにはどうすればよいですか?

mysql> Insert into year_testing (OrderDate) Values(CURDATE());
Query OK, 1 row affected (0.11 sec)
mysql> Select * from year_testing;
+------------+
| OrderDate  |
+------------+
| 2017-10-28 |
+------------+
1 row in set (0.00 sec)

mysql> Insert into year_testing (OrderDate) Values(NOW());
Query OK, 1 row affected, 1 warning (0.12 sec)

mysql> Select * from year_testing;
+------------+
| OrderDate  |
+------------+
| 2017-10-28 |
| 2017-10-28 |
+------------+
2 rows in set (0.00 sec)