There is a FAQ entry describing "How do I purge Piwik logs?" where the SQL statement contains "AND visit_first_action_time <= CURRENT_DATE() - 30".

That statement gives this error (MySQL server 5.0.51a):
"Incorrect datetime value: '20110293' for column 'visit_first_action_time' at row 1"

The statement should be corrected to:
"AND visit_first_action_time <= DATE_SUB(CURDATE(), INTERVAL 30 DAY)"

Additionally, it should perhaps also be stated that the tables have to be optimized afterwards to regain the diskspace that was freed in the DB.

PS: I tried to include the URL to the FAQ entry in question but my ticket is then rejected as spam. If possible please allow URLs for the piwik domain.