layouts/_default/sitemap.xml

{{ printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\"?>" | safeHTML }}
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:xhtml="http://www.w3.org/1999/xhtml">
  <url/>
  {{ range .Data.Pages }}{{ if ne .Params.sitemapExclude true }}
    <url>{{ $url := urls.Parse .Permalink }}
      <loc>{{ .Site.Params.SiteBaseURL }}{{ $url.Path }}</loc>{{ if not .Lastmod.IsZero }}
      <lastmod>{{ safeHTML ( .Lastmod.Format "2006-01-02T15:04:05-07:00" ) }}</lastmod>{{ end }}
    </url>{{ end }}{{ end }}
</urlset>