using SS4 date formatting http://userguide.icu-project.org/formatparse/datetime

code/GoogleSitemap.php

@@ -402,7 +402,7 @@ class GoogleSitemap
                         ->limit($countPerFile, ($i - 1) * $countPerFile)
                         ->last();
 
-                    $lastModified = ($sliced) ? $sliced->dbObject('LastEdited')->Format('Y-m-d') : date('Y-m-d');
+                    $lastModified = ($sliced) ? date('Y-m-d', strtotime($sliced->LastEdited)): date('Y-m-d');
 
                     $sitemaps->push(new ArrayData(array(
                         'ClassName' => $this->sanitiseClassName($class),

templates/GoogleSitemapController_sitemap.ss

@@ -4,9 +4,9 @@
 	<% loop $Items %>
         <url>
             <loc>$AbsoluteLink</loc>
-           	<% if $LastEdited %><lastmod>$LastEdited.Format(c)</lastmod><% end_if %>
+           	<% if $LastEdited %><lastmod>$LastEdited.Format(YYYY-MM-dd)</lastmod><% end_if %>
             <% if $ChangeFrequency %><changefreq>$ChangeFrequency</changefreq><% end_if %>
             <% if $GooglePriority %><priority>$GooglePriority</priority><% end_if %>
         </url>
 	<% end_loop %>
-</urlset>
+</urlset>