加算、減算、乗算、除算の演算子は、MySQL テーブルに格納されている日付値をどのように処理しますか?

mysql> select * from example;
+------------+
| orderdate  |
+------------+
| 2017-05-25 |
+------------+
1 row in set (0.00 sec)

mysql> select orderdate+10 from example;
+--------------+
| orderdate+10 |
+--------------+
|     20170535 |
+--------------+
1 row in set (0.00 sec)

mysql> select orderdate*10 from example;
+--------------+
| orderdate*10 |
+--------------+
|    201705250 |
+--------------+
1 row in set (0.00 sec)

mysql> select orderdate-10 from example;
+--------------+
| orderdate-10 |
+--------------+
|     20170515 |
+--------------+
1 row in set (0.00 sec)

mysql> select orderdate/10 from example;
+--------------+
| orderdate/10 |
+--------------+
|    2017052.5 |
+--------------+
1 row in set (0.00 sec)