mysql> set @t='2007-01-01';
Query OK, 0 rows affected (0.00 sec)

mysql> select date_format(timediff(now(),@t),'%Y-%m-%d %H:%i:%s');
+-----------------------------------------------------+
| date_format(timediff(now(),@t),'%Y-%m-%d %H:%i:%s') |
+-----------------------------------------------------+
| NULL                                                |
+-----------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------+
| Level   | Code | Message                                      |
+---------+------+----------------------------------------------+
| Warning | 1292 | Truncated incorrect time value: '2007-01-01' |
+---------+------+----------------------------------------------+
1 row in set (0.00 sec)