$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$_reviewsColFactory = $objectManager->get("\Magento\Review\Model\ResourceModel\Review\CollectionFactory");
$_storeManager = $objectManager->get('\Magento\Store\Model\StoreManagerInterface');
$reviewsCollection = $_reviewsColFactory->create()
->addStoreFilter($_storeManager->getStore()->getStoreId())
->addStatusFilter(\Magento\Review\Model\Review::STATUS_APPROVED)
->setDateOrder();
<?php foreach($reviewsCollection as $review):?>
<?php echo $review['review_id']; ?>----<?php echo $review['created_at']; ?><br/>
<?php endforeach;?>
$review['created_at'] is not getting the value that is present in database.