jekyll sitemap use site.time for lastmod so it doesn’t really reflect the actual last modified date of the post but the last updated date of the site instead.

I’m not so sure if lastmod attribute has much effect on Google’s crawl rate/SEO to your site since it’s optional but I’m a little picky even when it comes to small thing like this.

Since last modified date is available from the source file, it should be generated automatically from that. kinnetica’s sitemap plugin for jekyll make use of .mtime for lastmod in sitemap which is the actual last modified date of the post.

Clone the repo, put sitemap_generator.rb in your _plugins folder and you’re all set.