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How accurate are sitemap lastmod values? Bing's study

Bing's own study of sitemaps found 18% of lastmod values set incorrectly, usually to the generation date, and Google says it only trusts accurate ones.

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Written byAdam Hafez
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Key takeaways

  • Bing reported that 58% of hosts with at least one indexed URL had a known XML sitemap, and 84% of those sitemaps set a lastmod attribute.
  • Bing reported that 79% of lastmod values were correct and 18% were not correctly set, with identical dates across URLs the most common fault.
  • After talking to webmasters, Bing concluded most wrong dates were the sitemap generation date, not the date the content was modified.
  • Bing said it was rebuilding its crawl scheduling to use lastmod, so accurate dates reduce crawling of unchanged pages and favor recently updated ones.
  • Google documents that it uses lastmod only if it is consistently and verifiably accurate, which makes an untrustworthy date a liability.

Most sitemap advice says to include lastmod. The harder question is whether the dates in real sitemaps are worth reading. Bing’s webmaster team answered part of that in February 2023 with a short study of how sitemaps set the tag, and the result explains why search engines are cautious about it.

What did Bing measure about sitemap lastmod?

Bing looked at websites with at least one URL indexed by Bing. It reported that 58% of those hosts had at least one XML sitemap known to Bing. Of those sitemaps, 84% set a lastmod attribute. Of the sitemaps with lastmod, Bing stated that 79% had correct values and 18% had values that were not correctly set.

The post does not describe how Bing decided a value was correct, and it gives no sample size. Treat the percentages as Bing’s own reported figures, not as an audit anyone can reproduce.

Why were so many lastmod values wrong?

Bing named one dominant fault: identical lastmod values across URLs. After consulting webmasters, it found the date on most sitemaps was set to the date the sitemap was generated, not the date the content was modified. A generation timestamp changes every build whether or not a page did, so it tells a crawler nothing about which pages to revisit.

Bing’s advice followed from that. Set lastmod to the date the page was last modified, use the W3C Datetime format, and generate sitemaps at least daily so the tag keeps pace with edits.

Does Google use lastmod?

Google does not publish a study like Bing’s. Its sitemap documentation says it uses the lastmod value if it is consistently and verifiably accurate, for example by comparing it to the last modification of the page. That conditional is the practical takeaway: a sitemap that stamps every URL with a build date is the pattern Bing found, and it gives a crawler no reason to trust the field. If you cannot make lastmod track real edits, do not publish it.

What are the limits of this evidence?

The study is a vendor’s summary of its own data, published as a blog post. It shows how often lastmod is set badly, and that Bing changed its crawl scheduling in response. It does not measure how much faster pages are recrawled after a site fixes its dates, and it does not cover Google. Those gaps are why this piece reports the hygiene numbers and stops there.

The evidence

Sample
Hosts with 1+ Bing-indexed URL; size not disclosed

Hypothesis: Search engines can only use sitemap lastmod values to schedule crawling if those values reflect real content changes, so the share of sitemaps that set lastmod incorrectly determines how much of the signal is usable.

Method: Bing analyzed hosts with at least one URL indexed by Bing, checked whether each known XML sitemap existed, whether it set a lastmod attribute, and whether the values were correct. It then consulted webmasters to explain the incorrect ones. Bing did not publish the exact test it used to judge correctness.

Findings

  • Among hosts with at least one Bing-indexed URL, 58% had at least one XML sitemap known to Bing, and 84% of those sitemaps had a lastmod attribute set.
  • Of the sitemaps with lastmod, Bing reported 79% had correct values and 18% had values not correctly set.
  • The most prevalent fault was identical lastmod values across URLs, which webmasters said came from setting the date to when the sitemap was generated, not when content changed.
  • Bing said it was revamping its crawl scheduling stack to use lastmod, with a full rollout planned by June 2023, and advised generating sitemaps at least daily.

Limitations: Bing published percentages but no sample size, date range, or definition of a correct value, and the reported shares do not add up to 100 percent. The study measures sitemap hygiene on Bing-indexed hosts, not crawl outcomes, and says nothing about Google, whose only public statement is documentation, not data.

Sources

  1. 1.The Importance of Setting the lastmod Tag in Your Sitemap - Bing Webmaster Blog, February 1, 2023Primary
  2. 2.Build and Submit a Sitemap - Google Search Central

About the author

Adam Hafez
Adam Hafez

Founder

Founder, UpgradIQ, Inc.

Adam Hafez works on technical SEO and search measurement: how pages get crawled, indexed, ranked and now quoted by answer engines. He founded UpgradIQ, which reads Google Search Console and GA4 to tie ranking movement back to the changes that caused it. He publishes what the data supports and states the limits of it.

  • Technical SEO
  • Search Console and GA4 measurement
  • Answer engine optimization
  • Structured data

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