Sitemap lastmod is not a valid date
What this means
One or more lastmod values cannot be parsed as a W3C datetime. Google ignores lastmod it cannot read, so the file loses its ability to say what has recently changed.
How to fix it
Emit lastmod in W3C datetime format, either a date or a full timestamp with a timezone offset.
Fix effort
This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.
Authoritative source
https://developers.google.com/search/docs/crawling-indexing/sitemaps/build-sitemap
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