xml validation in php fails for sitemap file generated by xmlsitemap drupal module

am using schemavalidate() from php to validate my sitemap.xml file. This sitemap.xml file is generated by drupal module(xmlsitemap). When I run the schemavalidate, I get errors. Here is the code,

libxml_use_internal_errors(false);

$xmlDom = new DomDocument('1.0', 'utf-8');
$xmlDom->validateOnParse = true;

if(!$xmlDom->load(xml file location - url))
{
$errors = libxml_get_errors();
libxml_clear_errors();
$is_file_valid = FALSE;
}
else
{
if (!$xmlDom->schemaValidate('http://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd'))
{
$Errors = libxml_get_errors();
$is_file_valid = FALSE;
libxml_clear_errors();
}
else
{
$is_file_valid = TRUE;
}
}
I see the following error, Element '{http://www.sitemaps.org/schemas/sitemap/0.9}lastmod': '2011-03-07T01:53Z' is not a valid value of the union type '{http://www.sitemaps.org/schemas/sitemap/0.9}tLastmod'

Let me know, If I am missing something with validation or do I have to workaround with this error.

Note: If I generate sitemap.xml for my site online and then do a validation, I do not see any errors.

On my drupal pages I see this message,
warning: DOMDocument::schemaValidate() [domdocument.schemavalidate]: Validation failed: no DTD found !Element '{http://www.sitemaps.org/schemas/sitemap/0.9}lastmod': '2011-03-07T01:53Z' is not a valid value of the union type '{http://www.sitemaps.org/schemas/sitemap/0.9}tLastmod'.

I have PHP Version 5.3.5. Regards.