To get last finished date you can try this:

	$tasks = Mage::getModel('cron/schedule')->getCollection()
		->addFieldToSelect('finished_at')
		->addFieldToFilter('job_code', 'orders_exportsales')
		->addFieldToFilter('status', 'success');
	$tasks->getSelect()
		->limit(1)
		->order('finished_at DESC');

	$lastRun = $tasks->getFirstItem()->getFinishedAt();

---

__Bug:__

app/code/core/Mage/Cron/Model/Resource/Schedule/Collection.php

    Mage::getModel('cron/schedule')->getCollection()
        ->orderByScheduledAt($dir)

Sort direction is always set to `ASC` ... to fix replace

     $this->getSelect()->order('scheduled_at', $dir);

with

    $this->getSelect()->order('scheduled_at ' . $dir);