I would like to thank @Flip and @lifo for their great support in helping me resolve this:
$query = $this->getEntityManager()->createQuery("
SELECT DISTINCT(p.date)
FROM MainBundle:Price p
LEFT JOIN MomentumBundle:EmaPrice ep
WITH ep.price = p.id
WHERE ep IS NULL
ORDER BY p.date ASC
")
->setMaxResults(1);
$date = $query->getOneOrNullResult();