@@ -24,7 +24,7 @@ import { parseFrontmatter, summaryFromBody } from "./lib/frontmatter.mjs";
 import { listEntryFiles, idMatchesFile } from "./lib/entry-files.mjs";
 import { entryPageHtml, scriptJson } from "./lib/entry-page.mjs";
 import { LinkError, makeLinkResolver, caseMismatch } from "./lib/links.mjs";
-import { atomFeed, catalogCsv, homeJsonLd, sitemapXml } from "./lib/exports.mjs";
+import { atomFeed, catalogCsv, commitDatesFromLog, DATED_PATHS, homeJsonLd, sitemapXml } from "./lib/exports.mjs";
 import { schemaErrors } from "./lib/schema-check.mjs";
 import { llmsFullTxt, llmsTxt } from "./lib/llms.mjs";
 import { deprecationReason, MarkdownError, renderBlocks, renderInline, splitEntryBody } from "./lib/markdown.mjs";
@@ -222,33 +222,23 @@ function headMetaHtml(site, stats, generatedAt, hasCard) {
 
 /**
  * Repo-relative file => date of its last commit, from one `git log` over the
- * catalog and stacks. The sitemap's lastmod must never be earlier than the
+ * catalog, stacks and docs (DATED_PATHS). The sitemap's lastmod must never be earlier than the
  * last content change, and an entry's `verified` date is not that. Empty when
  * git is unavailable (a tarball, a shallow CI checkout with no history).
  */
 function lastCommitDates() {
-  const dates = new Map();
   try {
-    const out = execFileSync("git", ["log", "--format=%x00%cs", "--name-only", "--", "catalog", "stacks"], {
+    const out = execFileSync("git", ["log", "--format=%x00%cs", "--name-only", "--", ...DATED_PATHS], {
       cwd: ROOT,
       encoding: "utf8",
       maxBuffer: 64 * 1024 * 1024,
       stdio: ["ignore", "pipe", "ignore"],
     });
-    let date = null;
-    for (const line of out.split("\n")) {
-      if (line.startsWith("\0")) {
-        date = line.slice(1).trim();
-        continue;
-      }
-      const file = line.trim();
-      // Newest commit first, so the first date seen for a file is its latest.
-      if (file && date && !dates.has(file)) dates.set(file, date);
-    }
+    return commitDatesFromLog(out);
   } catch {
     // No git: the sitemap falls back to the build date.
+    return new Map();
   }
-  return dates;
 }
 
 function robotsTxt(site) {
@@ -491,7 +481,7 @@ function writeCategoryPages({ entries, config, stats, stamp, hasCard, now }) {
  * docs/ guides as guide/<id>/ pages. Links to entries go to their pages,
  * links to another guide go to its page, anything else to GitHub.
  */
-function writeGuidePages({ entries, config, stats, stamp, hasCard }) {
+function writeGuidePages({ entries, config, stats, stamp, hasCard, modified = new Map() }) {
   const written = [];
   const errors = [];
   const resolverFor = linkResolvers(entries, config, "../../entry/");
@@ -516,7 +506,7 @@ function writeGuidePages({ entries, config, stats, stamp, hasCard }) {
     };
     try {
       const rendered = renderGuide(fs.readFileSync(path.join(ROOT, guide.path), "utf8"), { file: guide.path, resolveLink });
-      writeOut(path.join("guide", guide.id, "index.html"), guidePageHtml({ guide, rendered, site: config.site, stamp, total: stats.total, hasCard }));
+      writeOut(path.join("guide", guide.id, "index.html"), guidePageHtml({ guide, rendered, site: config.site, stamp, total: stats.total, hasCard, modified: modified.get(guide.path) || null }));
       written.push(`guide/${guide.id}/index.html`);
     } catch (err) {
       errors.push(err.message);
@@ -695,15 +685,16 @@ function main() {
   const categoriesInSitemap = Object.keys(config.categories)
     .filter((cat) => visible.some((e) => e.category === cat))
     .map((id) => ({ id, lastmod: stamp }));
-  writeOut("sitemap.xml", sitemapXml({ site: config.site, entries: visible, stacks, stamp, modified: lastCommitDates(), categories: categoriesInSitemap, guides: config.guides }));
+  const modified = lastCommitDates();
+  writeOut("sitemap.xml", sitemapXml({ site: config.site, entries: visible, stacks, stamp, modified, categories: categoriesInSitemap, guides: config.guides }));
   writeOut("robots.txt", robotsTxt(config.site));
   writeOut("404.html", notFoundHtml(config.site));
   writeOut(path.join("freshness", "index.html"), freshnessPageHtml({ stats: fresh, site: config.site, stamp, total: stats.total, hasCard }));
 
   const pages = writeEntryPages({ entries, bodies, config, stats, stamp, hasCard, now, usedIn });
   const stackPages = writeStackPages({ stacks, entries, config, stats, stamp, hasCard, now });
   const categoryPages = writeCategoryPages({ entries: visible, config, stats, stamp, hasCard, now });
-  const guidePages = writeGuidePages({ entries, config, stats, stamp, hasCard });
+  const guidePages = writeGuidePages({ entries, config, stats, stamp, hasCard, modified });
   const pageErrors = [
     ...stackErrors,
     ...pages.errors,
 
@@ -5,6 +5,31 @@
  */
 import { esc } from "./shared.mjs";
 
+/**
+ * The folders whose files get a page with a lastmod: catalog entries, stacks
+ * and the docs/ guides. build.mjs runs one `git log` over them.
+ */
+export const DATED_PATHS = ["catalog", "stacks", "docs"];
+
+/**
+ * Repo path => YYYY-MM-DD of its last commit, from
+ * `git log --format=%x00%cs --name-only` output (newest commit first, so
+ * the first date seen for a file is its latest).
+ */
+export function commitDatesFromLog(out) {
+  const dates = new Map();
+  let date = null;
+  for (const line of String(out).split("\n")) {
+    if (line.startsWith("\0")) {
+      date = line.slice(1).trim();
+      continue;
+    }
+    const file = line.trim();
+    if (file && date && !dates.has(file)) dates.set(file, date);
+  }
+  return dates;
+}
+
 /**
  * sitemap.xml. Every URL carries a lastmod that is never earlier than the
  * last content change: the build date for the home and freshness pages, which
 
@@ -7,7 +7,7 @@ export function guidePageUrl(site, id) {
   return `${String(site.siteUrl).replace(/\/+$/, "")}/guide/${id}/`;
 }
 
-export function guidePageHtml({ guide, rendered, site, stamp, total, hasCard }) {
+export function guidePageHtml({ guide, rendered, site, stamp, total, hasCard, modified = null }) {
   const url = guidePageUrl(site, guide.id);
   const title = `${guide.title} | ${site.title}`;
   const description = clipText(plainText(rendered.firstParagraph) || guide.title, SEO_DESCRIPTION_MAX);
@@ -17,6 +17,8 @@ export function guidePageHtml({ guide, rendered, site, stamp, total, hasCard })
     "@type": "Article",
     headline: plainText(rendered.title),
     url,
+    // The doc's last commit, the same date the sitemap gives (build.mjs).
+    ...(modified ? { dateModified: modified } : {}),
     isPartOf: { "@type": "WebSite", name: site.title, url: site.siteUrl },
     breadcrumb: breadcrumbLd([
       { name: site.title, url: site.siteUrl },
 
@@ -546,6 +546,18 @@ eq("seo description fits the limit", seoDescription({ license: "CC-BY-4.0", comm
 
 /* sitemap lastmod (#108) --------------------------------------------------- */
 import { sitemapXml } from "./exports.mjs";
+{
+  const { commitDatesFromLog, DATED_PATHS } = await import("./exports.mjs");
+  const log = "\0" + "2026-10-20\n\ndocs/fonts.md\ncatalog/2d/a.md\n\0" + "2026-09-01\n\ndocs/fonts.md\ndocs/data.md\n";
+  const dates = commitDatesFromLog(log);
+  eq("git dates: newest commit wins", dates.get("docs/fonts.md"), "2026-10-20");
+  eq("git dates: older-only file keeps its date", dates.get("docs/data.md"), "2026-09-01");
+  const siteCfg = JSON.parse(nodeFs.readFileSync(new URL("../config.json", import.meta.url), "utf8"));
+  const undated = (siteCfg.guides || []).filter((g) => !DATED_PATHS.some((d) => g.path.startsWith(`${d}/`)));
+  eq("git dates: every guide lives under a dated path", undated.map((g) => g.path).join(", "), "");
+  const sm = sitemapXml({ site, stamp: "2026-11-03", entries: [], guides: [{ id: "fonts", path: "docs/fonts.md" }], modified: dates });
+  eq("sitemap: a guide's lastmod is its doc's last commit", sm.includes("<loc>https://x.test/s/guide/fonts/</loc><lastmod>2026-10-20</lastmod>"), true);
+}
 {
   const sm = sitemapXml({
     site,