New Post: Blog posts in the sitemap

I needed a sitemap for my blog also. What i came up with was this:

https://gist.github.com/s093294/5626622

Atm it in the lastmod just uses the Data property of the blog entity. Are there anyway to get a date for when it was last changed? Or should I extend the Entity Type with a property for when it was last changed and set this if I change a blobpost.
(I dont know much about SEO, so I just assume that if the lastmod flag have not been changed, then the search engine ignores the entry?).

it create a sitemap as this:

This XML file does not appear to have any style information associated with it. The document tree is shown below.
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>/Blog/2012/10/07/Hello-World---Who-am-I</loc>
<lastmod>2012-10-07</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
/Blog/2013/04/27/Fun-day-with-Global-Windows-Azure-Bootcamp
</loc>
<lastmod>2013-04-27</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
/Blog/2013/05/18/Staging-a-Composite-C1-Site-locally
</loc>
<lastmod>2013-05-18</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
/Blog/2013/04/18/Testing-and-Solving-a-Matlab-Exam-Solution
</loc>
<lastmod>2013-04-18</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>/Blog/2013/05/21/Tips-and-Tricks-for-CloudBerry-1</loc>
<lastmod>2013-05-21</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
/Blog/2013/04/26/Deploying-social-oauth2-claims-aware-websites-to-Azure-using-Git
</loc>
<lastmod>2013-04-26</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>/Blog/2013/04/13/Life-and-a-new-job</loc>
<lastmod>2013-04-13</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>
/Blog/2013/05/18/Composite-C1-CMS-for-Windows-Store-Apps
</loc>
<lastmod>2013-05-18</lastmod>
<changefreq>monthly</changefreq>
<priority>0.5</priority>
</url>
</urlset>