mysql> set @t='2007-01-01 00:00:00';
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 datetime value: '10579:13:15' |
+---------+------+---------------------------------------------------+
1 row in set (0.00 sec)


mysql> select time_format(timediff(now(),@t),'%Y-%m-%d %H:%i:%s');
+-----------------------------------------------------+
| time_format(timediff(now(),@t),'%Y-%m-%d %H:%i:%s') |
+-----------------------------------------------------+
| 0000-00-00 10579:19:00                              |
+-----------------------------------------------------+
1 row in set (0.00 sec)