Try this in your product collection

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