Try this in your product collection
$currentDate = Mage::getModel('core/date')->gmtDate('Y-m-d H:i:s'd');
$plusOneMonth = date('Y-m-d', strtotime($currentDate . "+1 months"));
$collection = Mage::getModel('catalog/product')->getCollection();
$collection->addFieldToFilter('expiry_date', array('from' => $currentDate, 'to' => $plusOneMonth));