public function isPublicationValid(ExecutionContextInterface $context)
    {
        // No start publication date specified for the end publication date
        if (!$this->getStartedAt() && $this->getEndedAt()) {
            $context->addViolationAt('startedAt', 'Start publication date is required when setting end publication date.');
        }
        // Start publication date is equal or later than end publication date
        if ($this->getEndedAt() && $this->getStartedAt() && $this->getStartedAt()->getTimestamp() >= $this->getEndedAt()->getTimestamp()) {
            $context->addViolationAt('endedAt', 'End publication date must be later than start publication date.');
        }
    }