I have an app with Doctrine 1 and I generate update_datetime fields for objects via new Zend_Date->getIso(). It worked just fine for years, but
This is caused by Zend not setting your timestamp format to one that matches what MySQL is expecting. You could disable STRICT mode in MySQL, but this is a hack and not a solution (MySQL will attempt to guess what the date you're entering is).
In Zend you can set the datetime format to what MySQL is expecting to solve this:
$log = new Zend_Log ();
$log->setTimestampFormat("Y-m-d H:i:s");