Jun 24
25
Google has made a small note to the sitemap documentation to add that the lastmod date specified in the sitemap file “should reflect the date and time of the last significant update to the page.” This change was made after I covered the topic of Google trusting or not trusting that lastmod field in your sitemap file.
See the rest here:
Google Sitemaps Docs Updated: Lastmod Date Should Be Last Significant Update