Microsoft Bing Says The lastmod Tag In XML Sitemap File Is Critical

Microsoft Bing posted a new blog post saying “for XML sitemaps, one of the most critical tags you can include in your sitemap is the “lastmod” tag.” And it will become even more critical as Bing is reworking its crawl scheduling stack to rely more on this lastmod field.

Yes, by June, the way Bing decides what to crawl will be more dependent on the lastmod tag. Fabrice Canel from Microsoft wrote, “we are revamping our crawl scheduling stack to better utilize the information provided by the “lastmod” tag in sitemaps.” This is being done so it can “enhance” the “crawl efficiency by reducing unnecessary crawling of unchanged content and prioritizing recently updated content.”

“We have already begun implementing these changes on a limited scale and plan to fully roll them out by June,” he added.

So making sure your lastmod date is accurate is now even more important. It should be the last time you modified the URL, not the time the URL was first published and not the time the XML sitemap file was generated. In fact, that is the biggest issue Bing found with the field, that it often just shows the date the XML sitemap file was generated and not the date the page of the URL was last modified.

Here are some data points Bing put together on XML sitemaps:

• 58% of hosts have at least one XML sitemap.
• 84% of these sitemaps have a lastmod attribute set.
• 79% have lastmod values correct.
• 18% have lastmod values not correctly set.
• 3% has lastmod values for only some of the URLs.
• 16% of these sitemaps don’t have a lastmod attribute set.
• 42% of hosts don’t have one XML sitemap

Oh, Bing still wants you to use the IndexNow protocol for the most efficient crawl solution but if you don’t – make sure your lastmod date is accurate.

In terms of Google, in 2015 Google said they don’t really use the lastmod date but then changed that in 2020 they said they do. The current Google documentation says, “Google uses the lastmod value if it’s consistently and verifiably (for example by comparing to the last modification of the page) accurate.”

Forum discussion at Twitter.