In a sitemap, is it important to include information about when the url was last changed in a <lastmod> tag?How does not including a <lastmod> tag in a sitemap affect how my site is crawled?If I leave this information out, how does it affect indexing in search engines?According to the protocol definition this is an optional tag.
How important is it to include  in a sitemap?
seo;sitemap;xml sitemap