Google reporting that your OpenCart generated sitemap submission has invalid dates? Here is a quick fix we applied to adjust the date problem directly in the products table…

UPDATE `oc_product` SET `date_modified` = `date_added` WHERE `date_modified` LIKE '0000-00-00%'

The products for the store in question were populated using a OpenCart-to-Miva importer developed in-house. We apparently forgot to include the modified date 😛