Microsoft Bing will soon rely much more on the date specified in the “lastmod” field in the XML sitemap for crawling.

The “lastmod” field is the date the page was last modified, which appears in the XML sitemap file. It is not necessarily the date the URL was created, but the date of the last time that page was modified.

This information is used by search engines to determine how frequently to crawl the website, and to determine which pages to index and ones to leave out.

Using the lastmod tag is crucial because it allows search engines to easily determine when a page was last updated, explains Fabrice Canel, principal product manager at Microsoft Bing

“Without it, search engines may delay crawling updated content or may over-crawl your website as they cannot accurately determine if the content has been modified,” Canel writes in a post published Wednesday.

A recent Microsoft Bing study reveals that among websites with at least one URL indexed by Bing, 58% of hosts have at least one XML sitemap, 84% of these sitemaps have a lastmod attribute set.

It also shows that 79% have lastmod values correct, while 18% have lastmod values not correctly set, and 3% have lastmod values for only some of the URLs.

Some 16% of these sitemaps do not have a lastmod attribute set, and 42% of hosts do not have one XML sitemap.

The biggest issue that Bing identified with incorrectly setting lastmod values in XML sitemaps is that they are identical. Microsoft Bing provides an example.

As it turns out, the date on most XML sitemaps was set to the date of generation of the sitemap, rather than the date of content modification. As a result, Microsoft made changes to sitemap.org to improve the clarity. The date must be set to the date the linked page was last modified, not when the sitemap is generated. Big difference.

And while it is possible for sitemaps to have the same dates, if Bing observes that the dates are consistently set to the current date, the engine will question the validity of those dates and may disregard them.

In the post, Canel provides details on how to set the lastmod and use the tag for Bing.

This information is used by search engines to determine how frequently to crawl the website, and to determine pages to index and ones to leave out.