MariaDB [test]> set sql_mode = '';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select timediff('2008-12-31 23:59:59.000001','2008-12-30 01:01:01.000002');

+---------------------------------------------------------------------+

| timediff('2008-12-31 23:59:59.000001','2008-12-30 01:01:01.000002') |

+---------------------------------------------------------------------+

| 46:58:57.999999                                                     |

+---------------------------------------------------------------------+

1 row in set (0.00 sec)

MariaDB [test]> drop table t1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> CREATE TABLE `t1` (`a` datetime DEFAULT NULL, `b` datetime DEFAULT NULL, `c` time GENERATED ALWAYS AS (timediff(`a`,`b`)) VIRTUAL ) ENGINE=MyISAM DEFAULT CHARSET=latin1;

Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> insert into t1 values ('2008-12-31 23:59:59.000001','2008-12-30 01:01:01.000002',default);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> select * from t1;

+---------------------+---------------------+----------+

| a                   | b                   | c        |

+---------------------+---------------------+----------+

| 2008-12-31 23:59:59 | 2008-12-30 01:01:01 | 46:58:58 |

+---------------------+---------------------+----------+

1 row in set (0.00 sec)

MariaDB [test]> set sql_mode='NO_ZERO_IN_DATE';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> drop table t1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> CREATE TABLE `t1` (`a` datetime DEFAULT NULL, `b` datetime DEFAULT NULL, `c` time GENERATED ALWAYS AS (timediff(`a`,`b`)) VIRTUAL ) ENGINE=MyISAM DEFAULT CHARSET=latin1;

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> insert into t1 values ('2008-12-31 23:59:59.000001','2008-12-30 01:01:01.000002',default);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> select * from t1;

+---------------------+---------------------+------+

| a                   | b                   | c    |

+---------------------+---------------------+------+

| 2008-12-31 23:59:59 | 2008-12-30 01:01:01 | NULL | <---- Notice "NULL"

+---------------------+---------------------+------+

1 row in set (0.00 sec)