I too saw this error on the latest version of the Wordpress SEO plugin in the sitemap index, specifically when listing the category sitemaps.
To resolve this problem temporarily I edited line 260 of the inc/class-sitemaps.php thusly:
I changed it from this:
$this->sitemap .= '<lastmod>' . htmlspecialchars( $date ) . '</lastmod>' . "\n";
to this:
$this->sitemap .= '<lastmod>' . htmlspecialchars( date("c",strtotime($date) )) . '</lastmod>' . "\n";
this makes it that whatever format the date was in, it is now reformatted into ISO 8601 format required by sitemaps.