sure this will work

SELECT * FROM `forum_topics` WHERE from_unixtime(topic_date) >= SUBDATE(NOW(),7)

See what SUBDATE does - it takes its 2. parameter as days by default. It also produces a datetime type in this case, you seem to just have a unix timestamp.

mysql> SELECT  SUBDATE(NOW(),604800);
+-----------------------+
| SUBDATE(NOW(),604800) |
+-----------------------+
| 0354-01-27 12:31:30   |
+-----------------------+

You\'d want

SELECT * FROM `forum_topics`
	 WHERE from_unixtime(topic_date) > SUBDATE(NOW(),interval 604800 second);

or

SELECT * FROM `forum_topics` WHERE from_unixtime(topic_date) > SUBDATE(NOW(),7)

or

SELECT * FROM `forum_topics` WHERE  topic_date > (UNIX_TIMESTAMP() - 604800);