collectionFilter.php

<?php
//Magento 2
\Magento\Catalog\Model\ResourceModel\Product\CollectionFactory  $productCollectionFactory,
        \Magento\Framework\Stdlib\DateTime\TimezoneInterface $timezone
$currentDate = $this->_timezone->date()->format('Y-m-d H:i:s');   
$collection = $this->_productCollectionFactory->create();

                $collection->addAttributeToFilter('expiry_date', array('or'=> array(
                    0 => array('date' => true, 'from' => $currentDate),
                    1 => array('null' => true))
                ), 'left');
                 
//Magento 1
$currentDate = Mage::app()->getLocale()->date()->toString(Varien_Date::DATETIME_INTERNAL_FORMAT);
$productCollection->addAttributeToFilter('expiry_date', array('or'=> array(
                    0 => array('date' => true, 'from' => $currentDate),
                    1 => array('is' => new Zend_Db_Expr('null')))
                ), 'left')