Sitemap contains invalid lastmod dates

A sitemap.xml file lists your site's pages for search engines, and each entry can include a lastmod date showing when that page was last updated. This check validates that every lastmod value is in the correct W3C datetime format. On this site, one or more lastmod values in the sitemap are not valid. Crawlers ignore lastmod dates they cannot parse, which means the freshness signal the sitemap was meant to provide, telling search engines which pages have genuinely changed and deserve a recrawl, is lost entirely for those specific entries until the format is corrected. Your developer should fix whatever is generating the sitemap so it emits dates in proper ISO 8601 format, for example 2026-09-10T00:00:00+00:00. This is usually a configuration fix in the sitemap generator rather than a manual edit, typically under an hour.