比较mysql / symfony2中的日期

I've look for this, and there's some related questions, but no one gives me the answer I need. I have an entity with a date field and I need to select those who are older than 7 days from now:

$query = $repository->createQueryBuilder('rf')
                    ->where('rf.sendDate >='.new \DateTime('-7 days'))
                    ->getQuery();

I getting this error:

Catchable Fatal Error: Object of class DateTime could not be converted to string

What I wonder is why it assumes that rf.sendDate is a string, when is defined as a DateTime objet in the entity? How I could compare this?

Any explanation really appreciated.

You should use parameters for this:

    $query = $repository->createQueryBuilder('rf')
            ->where('rf.sendDate >= :ts')
                ->setParameter('ts', new \DateTime('-7 days'))
            ->getQuery();