Sitemap plugin produces sometimes invalid timestamps for <lastmod> XML tag

[max-pfeiffer]

I noticed that this plugin produces invalid timestamps <lastmod>2025-11-22T11:17:34-00:00</lastmod>. -00:00 is not a valid ISO-8601 timezone. The proper value would be +00:00. See also the ISO-8601 wiki on that topic.

This seems to happen only for the standard pages like authors categories etc., not for articles:

<url>
<loc>http://127.0.0.1:8000/</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

<url>
<loc>http://127.0.0.1:8000/archives.html</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

<url>
<loc>http://127.0.0.1:8000/author/max-pfeiffer.html</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

<url>
<loc>http://127.0.0.1:8000/authors.html</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

<url>
<loc>http://127.0.0.1:8000/categories.html</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

<url>
<loc>http://127.0.0.1:8000/category/misc.html</loc>
<lastmod>2025-11-22T11:17:34-00:00</lastmod>
<changefreq>daily</changefreq>
<priority>1.0</priority>
</url>

Googles parser is very strict here. In Google Search console I see that my sitemap which is generated by this plugin could not be parsed.